I don't actually remember quite why I started using a trackball instead of a mouse. I made the switch during my first days at MacUser back in the mid-90s. A Senior Editor there had a Kensington TurboMouse, and when she left, I traded my mouse for her trackball. Perhaps it was my fondness for Crystal Castles that was behind the trackball envy. I don't know.
In any event, since then I've been a big fan of the trackball. I use a relatively small keyboard tray, and since the trackball doesn't actually move, it fits perfectly next to my keyboard. I like the larger arm motions I use when driving the trackball; I suspect it's one of the reasons I haven't succumbed to the RSI demon yet.
I've tried other trackballs -- gave Logitech's tiny thumb-driven ball a whirl at one point, only to discover it gave me wracking thumb cramps. Not recommended. In the end, I always came back to the Turbo Mouse.
Kensington's done a lot to the Turbo Mouse lately, including adding extra buttons to it and even a little scroll wheel. I never got the concept of a scroll wheel on a trackball, since there was no natural way to use it in the same way as on a mouse. (On a mouse, you can just rest a finger on it. But to drive a trackball you've got to move your whole hand.)
In any event, Kensington finally caught me in their trap. I tossed aside my Turbo Mouse for an Expert Mouse, which replaces the silly scroll wheel above the ball with a scroll ring around the circumference of the ball. It's a world of difference, and for the first time I'm using that scroll wheel technology the kids are so wild about.
The other great thing about the Expert Mouse is the fact that it's optical. Now, when I heard about optical trackballs I thought it was the silliest idea going. Optical mice, I understand, but why does it matter that you have an optical sensor lurking under a trackball?
Oh, but it does matter. The Expert Mouse glides more smoothly than any trackball I've ever used, even the one in that Missile Command console at the Round Table Pizza in Sonora, California. And there are no wheels to get clogged up with, um, trackball gunk.
Anyway, the Kensington Expert Mouse is my input device of choice. (If you'd like to share the identity of yours, you know how this works -- that's what the comments are for!)