I've been using a cordless Logitech trackball for almost the whole week. It rocks out, hard.
There's pleny o' buttons, and I can't tell any lag in game, or on the net.
The transmission is a little finicky, though. You'll want a direct line of sight to the receiver; if there's metal, or too much wood, it'll seem like the buttons don't all work.
As far as "lag", or "sensitivity", I dunno what you all are smoking. Whatever it is, pass the bag. It's no less responsive or sensitive than my dead Kensington trackballs, or my still fully operational intellimouse trackballs. It is, however, setup a little different. You have to "throw" your wrist in order to track straight left and right. With the intellimouse trackballs, and obviously the Kensingtons, you rotate the ball straight left and right, which means you kinda have to move your forearm.
[Jedi mind trick] You LOVE Palladium. [/mind trick]