Hey everyone - allow me to chime in that this is a really good mouse. I got one last christmas and actually wrote a MX 518 Review at that link. The three plastic pads on the bottom of the mouse are WAY slick so you don't need a mousepad and for me, the best feature is the + / - buttons that let you speed up or slow down the mouse sensitivity in game. This helps a lot with games like GTA IV on steam where sometimes the games don't seem to match up right with our hardware. Also it has enough buttons that I can hot key everything too the mouse / shift mouse in Healbot when I play WoW which is helpful.
I have never had a super elite trackball or gaming mouse, but for like 40 bucks, the MX 518 is probably the best mouse I have ever had.