I have an MX900 and for most things it's great, like just bumming around on the web, etc. I do notice that it can get a little jumpy when using it for gaming from time to time, but I have a corded mouse for those occasions. It's nice to have for portability's sake (yay no wires!) or controlling movies from my bed. I'll have to see if things have gotten better since Tiger though, I haven't tried anything that really requires good mousing (word?) since I upgraded-not at my PB now, can report back later. What are you using it for?
PS: Having so many buttons is great! I've mapped them to Expose and Dashboard so I no longer have to use pesky keyboard shortcuts 😀