I've only had ML installed on my Mac Mini (mid 2011) for a few days and so far, everything's been great. I haven't been able to perceive any big difference from Lion (what it came with) as yet. Safari seems to be a bit snappier, but I'm betting after my RAM upgrade gets here (ordered this 8GB kit from newegg), the differences will be that much more acute.
I did have some slight initial concerns regarding Office 2011 compatibility, but MS came through at the last minute and everything is gravy (though to be fair, Word is the only program I use here - excel & outlook get the most use on my older MBP).