Trackballs have ben around as long as mice have but somehow, mice seem to be the de facto method of input into a PC alongside the keyboard and the trackball has become a sidelined peripheral.
Which is a shame because this Trackman from Logitech is fantastic. When you get it out the box, it's clear that this is a quality method of controlling your computer. Obviously, it takes some getting used to but once you do, you'll wonder why you used to use a mouse.
The trackman is simple to setup - plug the receiver in and put the 2xAA batteries into the device and press the button on the receiver base and the trackman base to sync and you're away. Windows XP, Windows 7 and Mac OS X have no trouble using the device. Once it's working, it does make sense to install the Logitech software (not required to use) as this allows you to fully customise the 8 buttons to exactly how you want them as well as alter the sensitivity (and if you've never used a trackball before, you'll need to!)
Battery life is great with daily use, batteries can last over a month. You'll find you need to clean the ball every now and then as dust and grime builds up on the sensors but this is easily done by popping the ball out and just scraping away the filth - it's a pain but that's the downside to having you're greasy hands contacting the moving parts.
It's a large base and those with large hands should be quite comfy though those with tiny hands might want to consider a smaller device. In terms of stopping RSI, since I've had it, I've not had any issue with my wrists, where as I'd sometimes struggle with mice after a while.
The main downside is the cost - it's not fantastically cheap and I guess is due to the economics of scale (less people want them so less are made so it's more expensive). Also, the receiver is a bit big and ugly compared to the newer Logitech Unifying receiver (which this doesn't work with) but can be easily hidden.
Overall a fantastic device if you have limited desk space (you don't have to move it around) or for those that suffer from RSI. It's also a much nicer controller to use with CAD and other design software.