MX510 (red), but just got a Razer Diamondback Plasma LE just to see for myself why some people swear by it.
It'll take a few days (maybe weeks) for me to get used to the ambidexterous design again, since I'm so used to the right-handed Logitech MX5xx's (used a MX500 for a long time, then MX510).
I can't see for sure which one I like better, but the buttons on the Diamondback aren't as customizeable as I'd have hoped. There's 7 buttons, the traditional 3 and 2 on each side....but oddly enough, even with razer's mouse program, I can't set the 2 left side buttons to go back and forward in my web browser, like how the MX5xx's are setup. Since only the front-left side button can be set to "mouse4", which is usually the "go back" button....and the front-right side button can be set to "mouse5" which is usually the "go forward" button.
Pretty annoying problem.
EDIT: forgot to answer the original question of this thread, lol. Currently using a Razer Diamondback Plasma LE and Func 1030 mousepad (black)