I bougth the Microsoft Wireless 8000 mouse yesterday. I wanted something bluetooth so I wouldn't need to plug something into my macbook pro as it only has 2 usb ports. Plus it's rechargable AND matches my MBP perfectly!
I tried the BT MM but I didn't like how hard it was to right-click, it sucked for gaming. The 8000 has been working pretty good so far, had to go through some different steps to sync it with my computer but I am happy with it. Never lost the signal or had to resync it even while switching to windows.
I will admit it's not as smooth as the MM in OSX but it's not that bad. A little sensitive, but maybe it's something that takes getting used to. Hopefully Microsoft will release OSX drivers for the mouse. I know on their website they have an option to choose OSX but there's no download file for it yet.
Oh, and all the side-buttons and everything work fine. I have my side buttons programmed for expose.