I like mine, but two things you should know about them.
1. Until you get used to them, spinning hard to the left and a little down (8:30 o'clock), or up and a little to the right (12:30), the ball
might come a little out of the socket. (they are the spots exactly between the rollers) Once I replaced the original ball will an 8-ball pool ball, it "sat" in the socket better because the new ball is a little heavier. But the original ball now "sits" better too, so maybe it was because I was spinning it wrong.

2. (Important only if you are thinking of hacking one) They use a different setup than most mice/trackballs, and would be difficult to hack into a different casing.