I’ve noted quite a few people use track-balls instead of mice in this forum. Specifically, in the Palm Rests thread, I noticed @algernon and @fluffy both use them. I’ve been using a mouse my whole life so, I’m curious:
The specific trackball I use is the CST Laser Trackball with some custom modifications to make it not blind me. It’s a lot easier on my wrists than a mouse but isn’t as precise, although this particular mode does let you switch between three tracking speeds on the fly through a chording combination. It also has sockets to connect buttons or foot switches or whatever, which can then be mapped to interesting things in the OS although I haven’t used this functionality.
For more precise pointing I use a Wacom tablet, which I also sometimes use as a trackpad since mine is touch-enabled. So pretty much I’m always switching between different input modes based on what I’m doing, and I don’t even really think about what I use each one for (except drawing which is exclusively the domain of the Wacom).
In any case, I’ve always been more of a fan of trackballs than mice (maybe I played too much Centipede and Marble Madness as a kid?) and I find that mouse motions hurt my wrists more than trackball motions do. Also trackballs take less space overall and don’t get tangled cords.
Note that I only recommend finger trackballs and they need to also be fairly heavy so they don’t flop around; I do not recommend the Kensington Orbit, for example (too light), and I used to be a fanatic for the Logitech Marble-style thumb trackballs until I developed severe DeQuervain’s, which the ball probably caused or at least didn’t help with.
Also, this isn’t the case for my UI these days but I find that trackballs stack very nicely with “sloppy focus” (windows getting focus when you mouse over them), which in turn stacks really well with a tiling window manager. Back before I switched to macOS I had a pretty intricate setup which was great for this.
I just started using a Kensington Slimblade a few months ago and found the transition quite easy. I find it much more comfortable than using a normal mouse, where I’d started getting hand cramps and wrist discomfort. I find the trackball definitely not as dexterous as a mouse (if I play any sort of action game, I need to switch to a mouse (which has been very effective at making me not play games)) but not really a problem in practice, as my workflow is much more keyboard-driven (tiling window manager, live in emacs).
I started to use a trackball (Kensington Orbit, because of the scroll wheels) when I began to notice that my hand hurts when using a normal mouse. I wasn’t a big fan of the rodent to begin with, and preferred to do as many things with the keyboard as possible. This is partly because I’m a tad clumsy, and operating a mouse was often annoying. My coordination with it was abysmal, and I hated to wiggle it around on my desk. Especially when I had to cross large areas, which involved picking it up and placing it further away, just so I can pull it down again.
With a trackball, I have much more space on my desk, since I don’t have two wiggle a mouse around. My hands feel better, and surprisingly, I’m much more precise with it too! It appears that while my hand coordination is awful, my fingers are fine. 
Took me a few days to get used to the Orbit, but I have been using it exclusively since then, even for gaming (unless I play with a controller).
An interesting tidbit, is that back in the early 2000s, I helped a friend of mine run a computer lab, and they had Logitech Marbles there. I hated them, with a passion. I was reluctant to try a trackball, because of this. But somewhere along the years, whatever made me dislike trackballs, vanished. Now I love them.
Mind you, I’m not entirely happy with my Orbit, and will, eventually build my own. It’s going to be a long process…
I use a Microsoft Trackball Explorer on my gaming PC and a Logitech M570 everywhere else, including my gaming laptop while traveling.
My family has a history of being prone to RSI, so since 2000 I’ve used split keyboards (mostly of the MS Natural series, but now the Model 01) and trackballs (as described above) to minimize my personal wear-and-tear.
I recall that it took a little while to get used to a trackball, with a Trackball Explorer being my first one. I prefer ones with a fingertip ball to ones with a thumb ball, but I’ve used both on a daily basis for ages now and don’t have to think about it at all.
I wanted to like the Kensington Orbit because of the scroll wheels they just felt too awkward to me in practice.
Everything else aside, I love how portable the Logitech M570 is. It’s small enough that I can comfortably cup it in my hand against my leg while sitting down and use it in tight spaces (e.g. in an airport). Also, the AA battery lasts a long time (listed as 18 months).
Huh, your Orbit looks totally different than mine. Did Kensington reuse the name for different models of trackball?
I have what I think is the original Orbit, with two buttons and a scroll ring. I really dislike it for a bunch of reasons, mostly that it’s so lightweight and moves around on my desk.
Have you looked into CST? I absolutely love mine. I’d recommend against the “laser” version though; I had to modify mine to make it not constantly blind me out of the corner of my eye. (Do not stare into beam with remaining eye…) My main modifications being sanding the ball a bit to give it more of a matte/translucent finish, and applying black permanent marker to the ball-facing halves of the LEDs to reduce the brightness.
That’s how mine looks as well.
I placed it on a mouse mat, that seems to have worked. It only moves around when I place it. 
I did (and looked into a lot of others too), but settled with the Orbit due to the scroll ring. The scroll thingy on the CST doesn’t look like I’d like it. It is also a lot more expensive than the Orbit. I liked the Orbits form factor, the scroll ring, and its price. I got my money’s worth, easily.
The next one, the perfect one, I’ll build myself. Will likely borrow ideas from here and there…
Yeah the main complaint I have with the CST is that the scrollwheel doesn’t have ‘notches’ but instead scrolls smoothly-continuously. It took some getting used to although I don’t mind it now. I love its placement though, and I prefer it over the Orbit’s. Tastes vary.
The picture of the trackball in your photo in the “keyboard layouts” thread looks like the Kensington Expert which is why I thought you might have a different thing of the same name.
I usually use a Kensington Expert Mouse at home and a Slimblade at work.
I also have a CST, @algernon 's point about the scroll wheel is valid for me, and why I nearly never use it. There is no resistance when scrolling, and as a result, I found it difficult to be precise with. The placement also bothers me; it is hard to reach. (also: my eyes! the goggles - they do nothing! maybe I should try that light attenuating mod too…)
While I dislike the buttons on the slimblade, the twist-to-scroll is amazing. With the Expert mouse, the buttons are better, but the scroll wheel is slightly worse. It evens out though; any good-button-requiring-gaming will happen at home.
Full disclosure: I use i3, so my preferences for pointing devices are that they are unobtrusive, and used seldom in an average coding session.
For OP’s questions;
Better ergonomics for me, less wasted desk real estate, arguably better precision (noticeably better with things like marquee selection or sniping in an FPS)
A lot smoother than the transition from a Topre to a Keyboardio 
I think the choice of a trackball with a large ball makes it pretty seamless. It took a little while to neurally remap the left/right click, but actually pointing at on screen stuff took no time at all.
tldr;
trackballs > mice
kensington would get my recommendation.
I recently developed a lateral epicondylitis from rock climbing and probably also from mouse work. That’s why I am also highly interested in alternative solutions (trackballs, etc.). The information provided here is therefore of great value to me, thanks!
One thing I recently stumbled upon (slightly out of topic here) is a Precision Gaze Mouse. It is a new promising approach that combines eye and head tracking to gain fast and also precise control over the mouse pointer. If I wasn’t a Linux user, I would give it a try right away. Currently it is only supported on Windows. The way it is presented, it uses a camera for head tracking and I am not comfortable with such a solution. But the description also mentions IR based head tracking, which requires to wear reflecting stickers, e.g. on a headset.
I’m a longtime trackball user - Logitech M570 is my current model. When I’m in the office I have to use a traditional mouse, and I find it just much slower to move to and use without looking, and it takes up much less space on my cluttered/messy desk. So much better ergonomics, and easy to switch to once you’re used to it.
I rather like the look of the Precision Gaze Mouse though - I shall investigate!
I switched to an Aukey vertical mouse when I started getting wrist problems (tendon pain).
It worked fine, but now that I have a split keyboard (M01) I couldn’t resist buying a Kensington trackball - I’ve been using it for, maybe 2hrs now and I love it!
What always annoyed me was having to move my whole hand and arm quite far to the right, beyond the keyboard.
I’m not yet moving smoothly, due to all that new input hardware but it feels like being able to rest and relax my arms and hands a lot more, which is really nice.
ADDITION: Now, 3d later, I can say that another nice aspect of the trackball for me is that a) I can use both hands - one hand moves the ball, the other one does the clicks - that feels kind of “laid back”… plus b) it’s a symmetric use of hands and arms! Still loving it.
It’s great to hear about everyone’s various experiences! I’ll try to see if I can get a used one and give it a whirl.
Yeah the scroll-roller took a lot of getting used to. I’ve never really cared about precision in my scroll gestures since I’m just using it for rough scrolling on webpages or whatever, and usually use page up/down when working on code. Now I’ve come to really like the low-friction rolling motion, for what it’s worth.
Ok, well you’ve at least given me something to occupy myself with on a lazy sunday afternoon…worth re-evaulating, especially considering how different things feel with the keyboardio.
@algernon @fluffy currently I have a Logitech trackball setup sideways and trying to sense if trackball is the right thing for me (friend of mine lend me the Logitech). Thinking about the orbit but I have been afraid that by putting the trackball between the keyboardio will make me use it in a non natural position, meaning, having my wrist bend to the right (if using right hand) or ill just be using the trackball diagonally which will be funny for pressing the buttons. Currently I’m using only the track ball with right hand and using fn+mouse keys on keyboardio with left which seems that will start getting painful at some point due to the of constantly using thumb plus indicator (and holding)
What are your thoughts about it? Because as I see it the trackballs seem best used from an ergonomic point of view at the side keyboard for a more “natural” arm-wrist position.
Picture for reference.
Currently I have it with the track ball up, squeezed between the two parts to make it similar to the “normal” trackballs(trackball facing up) . One gotta make do with what he has 
So, I wouldn’t recommend that Logitech trackball at all, for reasons I already stated in this thread. And that placement of that trackball is particularly bad for the reasons you stated, although you could mitigate it by rotating it to the left a bit. Remember, you don’t HAVE to arrange things like they appear in product photos.
A finger-driven trackball like the CST or the Orbit would be fine in that position, because you can keep your wrist straight while working it (although I’d still recommend turning it so that it matches your hand’s natural position).
I used the CST in the central position for a while (particularly when using the Kinesis Freestyle) but with the Model01 it was way more comfortable to keep it to the right, since that provided better spacing between the keyboard halves with respect to my shoulder width and so on. The great thing about this setup is you can experiment and try different things to see what works the best.
Many thanks for your feedback 
How about the usage of the buttons on the CST or orbit if you keep it straight doesn’t it seem odd or uncomfortable?
Another Kensington Expert Mouse lover here (don’t let the name fool you; it’s a trackball). I’ve used them for so long I don’t even remember how they compare to anything else. I have one between the two halves of my Model 01, but I have the halves farther apart than @tacf shows in the picture above, and the Kensington turned about 45° counterclockwise, so the base is pointing towards my right hand. I find this arrangement entirely comfortable.
Mind you, I’m not entirely happy with my Orbit, and will, eventually build my own. It’s going to be a long process…
I’ve also been looking at building my own trackball, as switching to mouse and back with the keyboardio is getting me down. In my ergo mockups, it seems pretty feasible to have a thumb ball + a couple buttons that you can use just by pivoting your wrist inwards from the home position on the keyboardio, without actual significant wrist movement.
I’m also reminded that there’s a convenient expansion port on the board - I don’t think the pinout has been published as of yet, but if there’s a couple analog pins available and the ability to hook up another SPI device (for a PMW3360 or similar), there’s some nice benefits in being able to easily reuse KB buttons for mouse mode switching (for scrolling or similar) as well.
