I'm very attached to my Microsoft Trackball Optical - say what you will about their software, but Microsoft's keyboards/mice/trackballs etc are good. It was recognized and worked perfectly the minute I decided that the Pro Mouse was pretty and sleek and comfortable but basically annoying and replaced it.
On top of that, the Intellipoint software lets me program the various extra buttons on a global basis, or on a per-application basis. The combination of this and my Griffin Powermate is very nice indeed.
If the trackball has a problem, it's a tendency to accumulate Mouse Crud (tm) far more quickly than a mouse does, since your thumb is in contact with the ball most of the time. Even the "no moving parts" Pro Mouse isn't free from Mouse Crud (tm), that clear plastic really shows where your hand rubs against it.