OK, so I just got mine today, and it's all hooked up and ready to go. First impressions:
First of all, the cord is short. If your tower isn't right beside your workspace you're going to need a hub. Due to circumstances beyond my control, my tower is on the side of my desk opposite from where my trackball sits. Hub it is, then.
Second, I love the dimensions of the thing. It doesn't require the "eagle claw" grip that I had to use with my old Trackman. The primary buttons are large enough to accommodate hands of all sizes, as they extend from below the ball to above it. The middle button takes a bit more effort to activate, but that suits me fine. I just mapped it to the Browser Back function because I'm too lazy to click at the top of the screen.
The scroll wheel was catching a bit when I first started using it, but it seems that step number one to fixing problems with this trackball is "spin it real fast and see if that helps." In this case it did. I love that the scroll wheel doesn't click, and I wonder how I ever put up with it on my old Logitech.
So far the ball has a lot of friction in the movement. I'm hoping that this goes away with use. As it stands, it is very hard to move the cursor in a straight line. It has a natural tendency to curve.
Anyway, that's all I've got for now. Aside from the friction, I'm loving this trackball so far.
Quick edit: After rereading Ripster's L-Trac review, I switched the DPI to maximum and then messed with the tracking speed in the control panel until it felt right. Much easier to keep the cursor going in the direction I want now, although it still fights me a bit.