My upgrade to Mountain Lion went smoothly. As always, you should have a complete backup before installing a new OS. Here are a couple of links that should tell you everything you need to know about Mountain Lion:
The Ars Technica review, written by John Siracusa: http://arstechnica.com/apple/2012/07/os-x-10-8/
Where Lion stumbled, Mountain Lion regroups and forges ahead.
The Macworld Review by Jason Snell: http://www.macworld.com/article/1167804/mountain_lion_apple_gets_its_operatin...
Like Lion, Mountain Lion offers numerous feature additions that will be familiar to iOS users. This OS X release continues Apple’s philosophy of bringing iOS features “back to the Mac,” and includes iMessage, Reminders, Notes, Notification Center, Twitter integration, Game Center, and AirPlay Mirroring.