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i can recommend NOT to download Mountain Lion at this time, wait until the first udate(s) are fixing all the bugs coming along these days. I am suffering of the download, can't really use my computer now, no tone available anymore, always slow running now (have 8GB RAM), apps not running well, you always need to foce quit as software or programs are getting hung every now and then etc.
I normally wait with new uploads, this went fine over the last years, but a friend of mine got the Mountain Lion upgrade on his MacBook Pro and he told me it is running fine, so I gave it a try too, I got stuck right away, my friend has the same issues now after a week, it's running you mad, no help on the www, we need to wait for apple to fix all this issues first and release the upgrade...
I always wait for a little while before I upgrade the machines I rely on for work to most things when they are first released, whether that is software or hardware. I think that is just being prudent. There are always going to be some issues, whether major or minor.
That said, two people who I can literally see right now have already upgraded to Mountain Lion without any problems and are happy with it. That is obviously not exactly a scientific survey, admittedly. For myself, as noted above I would wait before updating on mission critical machines, but otherwise I would probably go ahead and update.