I downloaded Mountain Lion on the date of release onto my mid-2008 17" MacBook Pro. As of now, I will say I regret installing it. My computer is not reacting well from the new download and while I do think that software updates will me available in the coming weeks, the side effects of installing ML make it such a pain to use.
Here are some of the symptoms my MacBook Pro has. Boot up time lags. It takes about sixty seconds to boot to the log in window, then another 45 seconds. It was previously about half as long with Lion. Application launches lag. Safari and iTunes top the list, both taking about one minute to launch. All other applications lag too upon launch; about twice the amount of time it previously took to launch an application. Safari performance *****. Loading Facebook takes 30 seconds! Clicking onto a friend's profile takes 30 seconds to load! This is the most frustrating out of anything. Also, Skype video no longer works. I had to update Skype because of some type of software issue from the install of Mountain Lion. Switching between applications typically results in the rainbow swirling ball for about 15 seconds (ie: Safari to iTunes). Spotlight searches are much slower as well, and it seems like it is constantly loading new results. Overall system performance is noticeably sluggish in comparison with OS X Lion.
Positives of Mountain Lion? I noticed that graphics and video is a little sharper. I always appreciate that. Also some of the new apps included are cool, although I haven't really used them. With notes syncing to my iPhone, I thought I would use this convenience more, but it isn't something I use.
Anyway, Mountain Lion has potential. Apple needs to release an update soon though. This version of Mountain Lion is horrible. I don't think Steve Jobs would appreciate the quality of this software and all the negativeness associated with system performance.