Assuming your software and hardware are compatible with Mountain Lion, it’s a no-brainer upgrade. For $20, you’ll get a nice set of new features. Before you upgrade, check the following for compatibility:
Supported Macs and other requirements: http://www.apple.com/osx/specs/
Application compatibility: http://roaringapps.com/apps:table
Also, check with your printer manufacturer to see if your printer driver is compatible with Mountain Lion.
Rather than rehash the list of new features in Mountain Lion, I’ll refer you to a couple of comprehensive reviews: