John Siracusa, Ars Technica
The Mac is a platform in transition. In Lion, OS X began shedding the well-worn trappings of traditional desktop computing at an accelerated rate. This trend continues in Mountain Lion. Where Lion stumbled, Mountain Lion regroups and tries againwhile still forging bravely ahead in other areas.
As the second major refinement-focused release, it's easy to view OS X 10.8 as "what 10.7 should have been." The flip side of this argument is that the real-world mileage weve all put on Lion has helped Apple make the right kinds of adjustments in Mountain Lion. If we'd had to wait for two years after 10.6 for the next major release of OS X, chances are good that the worst of the missteps in Lion would just be landing on our doorsteps today. I'll take 10.8, thanks.
Jason Snell, Macworld
All told, I found Mountain Lion to be a stable, solid release. Even prerelease builds were far more stable than Ive come to expect from OS X betas, leading me to wonder if Apples new annual schedule is leading to more careful incremental updates (with fewer bugs) rather than great leaps (with more, nastier bugs).
Nilay Patel, The Verge
Ultimately, this is pretty easy: you should spend the $20 and upgrade to Mountain Lion, especially if you have a newer Mac. Youll gain a handful of must-have features, and everything will get faster and smoother. I havent really missed Snow Leopard at all since upgrading, which is remarkable considering how much I disliked Lion.
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http://www.macrumors.com/2012/07/25/roundup-of-os-x-mountain-lion-reviews-faster-and-smoother-incremental-bargain-at-twice-the-price