Well, my first choice was the Kensington Orbit trackball, based on reviews on this site. All-in-all, it is a great product, but it has 1 tragic flaw which killed it for me - it gunks up incredibly quickly; I am talking on the order of every hour or two. I returned it to the store from whence it was purchased (hint: the one with a big yellow tag). Strike one.
My next choice was what is essentially a corded version of the Logitech Cordless Trackman FX Optical Trackball, which was probably simply called the same thing, minus the "Cordless". The up side - it has several buttons; this device did nothing about my wrist cramp problem, however. Strike two. (Side note: I don't really see the point of getting a cordless trackball or keyboard, unless you are just dying to have more battery-powered things.)
Choice number three: the Logitech Marble Mouse (which is a trackball, despite its name). I just plain could not get used to the feel of it and it did very little to ease strain anyhow. A lot of people on this site have good things to say about it. I, however, do not. Guess the third time is not always a charm.
Well, I finally ended up with (you guessed it) the Logitech Trackman Wheel Optical. While it takes some getting used to, once you get a feel for this, you will never even want to go near another mouse. It has two buttons and the scroll wheel which also doubles as a third button when pressed in, although I never find myself in need of this third "button" myself. This is probably the most ergonomic pointing device you can hope to get...that is, assuming you are right-handed. Sorry, lefties, looks like your choices are considerably more limited when it comes to trackballs.
While being an optical device, the ball bearings upon which the trackball itself rests do tend to need cleaned off once or twice a day, but this only takes a few seconds and is not nearly as time-consuming as cleaning off the roll bars on the non-optical Kensington Orbit, and these cleanings aren't nearly as frequent, either. I don't think you are going to be able to buy a trackball that doesn't need cleaned fairly often, and this one does a nice job of making it a relatively painless affair.
The software allows you to set the directional orientation (which way is "up"), among other things, though I have not even bothered installing the software since I started using Windows XP.
Bottom line: If you are right-handed and have your own computer, buy this. Just make sure you give yourself a day or so to adjust to using it. You shan't be dissapointed.
I finally got tired of cleaning the ball on the old Trackman. My wife has both an Logitech marble trackball and a Logitech optical cordless mouse, and I wanted the best, too.
While each new Microsoft OS has had problems with the older version of Mouseware software, in my experience, Logitech is always the first to have updated drivers available shortly after release of new OS's.
After using the trackball for about 5 months, I'm still thrilled. Sure, the gunk from my oily, sticky, food-covered hands still builds up on the ball, but unlike mechanical mice and trackballs, it doesn't cause sticking and jumping of the cursor. The action was still smooth. I had not even noticed that it was getting harder to move the ball until I popped the ball out, and gave the contact points a quick cleaning.
So even with five months of built-up grime, it was still working just fine.
Now not everyone likes trackballs. I use a mouse at work, and it's probably better for me to be moving my whole arm. But at home my desk is too messy. There's no room to move a mouse, and even if I were able to, I'd hit my elbow on something else.
Not for one minute have I regretted buying this trackball.