My main mouse is a Logitech G9, but I had to RMA it with Logitech because there was something wrong with the laser. Logitech is replacing it with a G9x, which is awesome of them. I am using the mouse with the larger ID Grip that they have unfortunately stopped making. I have large hands, and with the ID grip, the G9(x) is the best mouse I have ever used. I hope they never stop making it, or at least something like it. It's just the best balance between form and function in a mouse I've seen.
As I'm waiting for my replacement G9x to come back, I'm using their G500 which I find very difficult to get used to. The laser isn't centered, and it's not balanced like the G9 is, but the tracking is very good.
I've used a ton of different mice: Razer DA, Intellimouse Explorer, etc., but I keep coming back to the G9.