Well, they've done a remarkable amount of improvements in the selection tools, particularly the QuickSelect and Refine edge interface, along with more intelligent keyboard control over brushes.

They've also improved the Photomerge and Auto-align features, and finally have the Video rendering in a form that takes advantage of all the regular PS features, particularly masking and filtering.
It's also seems to have been optimized for the newer OS but I've been reading about a lot of people experiencing a slow down when they've installed CS6.

If you're only doing color management, color corrections and some masking/compositing or filtering to images then there's no need to upgrade.