I just got the Steelseries Sensei Fnatic edition (since I heard it would be Mac compatible, and looks awesome). Great mouse overall, but I was a bit bummed about the lack of drivers / tools to configure the extra buttons, as mentioned above.
I looked around a bit, tried steermouse and usb overdrive, but those didn't detect it, and finally found ControllerMate - a program to remap mouse / keyboard / joystick buttons and program your own actions (make buttons "autofire", and tons of other neat functions for gamers). It identified the mouse and also a "Steelseries Keyboard" on my system (even though I didn't have one). I noticed that the left buttons registered as "MouseButton 4" and "MouseButton 5", but the right ones as "SteelSeries Keyboard PageUp" and "SteelSeries Keyboard PageDown". Anyway, it was easy to hook actions to the button and remap them to keyboard shortcuts, and now I have all buttons on the Sensei working while gaming

Just tried it with WoW, and all the side buttons are working to trigger actionbar items and such. Couldn't be happier!
Just a tip for anyone looking for a working gaming mouse for Mac. Also, the settings available in the display on the mouse itself are a great way to configure sensitivity and such.
Disclaimer: I'm in no way affiliated with the people producing / selling ControllerMate, just a happy user. The program said it would only work for 20 minutes while unregistered, but I've run it now for 1h+ and it's still remapping my buttons correctly.. I will probably buy it anyway, it's a great program overall, and only $15.
EDIT: Also, to leftCoaster above: I upped the polling rate from 125 to 1000, and I get no lag what so ever. Perfectly smooth