I have both Mountain Lion and Windows 7/8.
Mountain Lion >> Win8 for usability.
The start screen sucks big time. Move your mouse out of the way? Oops, charm comes up. The desktop, to be usable, has to have all the programs you use pinned to the task bar, else you’re dropping in and out of the metro interface to start anything. Some people I know hunt out the EXE files to run or put shortcuts on their desktop to avoid that.
Stability wise – its OK and under the hood there seems to have been improvements. I’ve never had ML crash on my 2009 iMac, but neither have had win8 crash either. Win7 on the other hand does tend to crash on shutdown of my work laptop (think its a driver issue).
Live tiles are nice when you first see them then shortly afterwards you ignore them. The Apple version of having, say, a little red number next to your mail program icon is better: you know something is there for you to look at but I can’t be faffed with sitting and waiting for a tile to update with something, usually a mail from CRC about their latest deals.
But lets face facts – the only reason Metro was put onto windows was the marketing department, to make all Windows offerings look the same, regardless of use. Win Phone 8/X-Box/Windows now all use metro and I know one gamer who doesn’t use his X-Box as much as he used to because of the change-for-change’s sake.
Metro is great for touch, but not much else.