Twitter is too short to go into this properly so I'm making an entry out of it. Yesterday I upgraded to Mac OS X 10.8 Mountain Lion. Here's some of my first findings with it. Like I have said in a tweeted response I think it's a bit too early to jump to conclusions as to the performance and how well OpenGL works because much of the performance increases I may think I'm experiencing may be chalked down to placebo. Just like how new OS updates tend to feel faster at first until the honeymoon period is over.
However that said it seems the performance is pretty competent so far and perhaps a little higher on average than 10.7 Lion. In one game.. Serious Sam 3 BFE I have noticed improved shadow rendering at a distance... previously it was grainy at a distance like a screen door and then it popped into full detail. No longer on Mountain Lion. SS3BFE was a game that was notorious for causing kernel panics and lockups on Macs when it first came out and even after they patched it later on I would still get a lockup or two on one or two parts of the game I still have to do a lot more testing to see if that's the case in Mountain Lion with the new graphics drivers.
Trine 2 seemed to work rather well at the default settings it detected for my computer. That's 1080p, high and FXAA. I hadn't played it much in Lion though so I'm not sure how much better it is now. I only briefly played the demo there.
One game tested so far seems to be not working as well on Mountain Lion though and that was my unofficial cider port of Star Trek Voyager Elite Force. It as getting choppy in Mountain Lion for some reason. I had to move that over to wineskin for it to work properly again. I hope other wrapped games like it don't exhibit that problem. I tried my unofficial X-Men Origins Wolverine port and it didn't so there's some hope not all my cider ports are affected that way.
I haven't tried all my games yet though so there's still room for updates on this.
As far as general use goes I like the OS but I'm missing candybar which is now broken in Mountain Lion. I'm not sure I like how the notification center looks but I'm sure it'll get handier all the time. I setup twitter with it but I haven't used it much as I'm used to just using a separate client like tweetbot to tweet. I like some of the small touches Mountain Lion has like the new auto birthday thing in calender. I'm not sure if I'd say the whole OS is snappier in general but Safari certainly seems to be faster now.