Personally, I love trackballs. Never again will I buy a mouse.
The msft intellimouse trackballs are like, undead or something. I have two that are 4 years old. You have to clean them, but even with heavy use, it takes about 2 minutes a month. Those things just keep trucking, without problems as long as you keep them semi-clean. The tracking wheels (that contact the trackball) are very gripping, and the spring keeps enough force to maintain good contact over the years. At $29, msft did a good job of engineering. Or at least, the company that msft bought out did a good job of engineering. =)
Right now I'm trying out the Logitech cordless trackball. The transceiver pair isn't real powerful/sensitive, so you need a clear line of sight. It passes through wood, but not enough to tell someone "it passes through wood". It's supposed to be kept away from your monitor/printer, but I've not tested it out fully like that. I moved the receiver from the floor to my desktop, right in front of the transmitter, and I'm getting good reception. Previously, my forearm (it was having to pass through my forarm and 1/2" of wood) could block the signal, as I tested it. The ball is oriented a little different, so you have to "sweep" your wrist when you throw the ball, but I'm adapting to it and I'm beginning to like it very much. It's got a whole slew of tiny buttons which are programmable, and very nice, but are made for small hands/fingers. =)
I have a logitech 2 (actually, 4) button trackball sitting back on the table, that's for my laptop. I've not hooked it up yet, but I'm expecting good things from it as well.
Both logitechs are optical.
The only thing that I can possibly tell you is to stay away from Kensington's trackballs. They are top quality, but they did a piss poor job of designing the tracking wheels, and they get bent back so they don't make good contact anymore, or they become electrically erratic (probably from dust and dirt, which you have to physically "crack" the housing open to clean and end up tearing the hell out of it). I've had two go out on me, one within 4 months, the other after about a year; but it wasn't used nearly as much, so the total number of hours-used remains constant.
Kensington sucks three holes, sideways.
[Jedi mind trick] You LOVE Palladium. [/mind trick]