I've purposely kept my iPad on 5.x because iOS 5 maps is the only iPad maps application that works well with public transit routing. iOS 6 maps doesn't do public transit at all, Google Maps and all the good third party transit routing apps are iPhone only.

Also a bunch of the new iOS 6 features (like Passbook, photo panorama, certain geofences, and the updated phone app) are not available at all on iPads. Siri (maybe?) and IPv6 over LTE are the only iOS 6 features I feel I am missing on the iPad so I am in no huge hurry to upgrade.