As of today, OS X 10.8, also known as Mountain Lion, is available. (At the time of this posting, it is not yet available, but should be in a few hours.) This latest version of OS X builds on the previous version, 10.7 Lion, notably adding a number of features inherited from iOS. Unlike earlier versions of Mac OS X, this release does not add any groundbreaking features, but rather refinements. It brings the desktop and mobile operating systems closer together, changing the names of some of the utilities on the desktop to match those in iOS, and adding some features that have proven successful in iOS. This is a controversial topic; some people lament the mobile-to-desktop changes, but I see the logic in what Apple is doidng.
I have been running Mountain Lion throughout the various developer releases, and when the gold master was released 2 weeks ago I installed it first on my laptop, then on my main Mac, a Mac Mini. It has proven very stable, even though there are some bugs, and some quirks. It is likely that there will be an update very quickly, as has been the case with previous versions of Mac OS X. But as it stands now, Mountain Lion seems solid enough for serious use. It is entirely possible that some software is not compatible, so if there are specific programs you absolutely need to do your work, make sure to check for compatibility before upgrading.
Mountain Lion is available from the Mac App Store for only $20.