I've been into caching for a few months now and started to place a few caches myself. I initially bought the gps for my hiking/backpacking but got turned on to this extra activity. It keeps me active, I've travelled up and down valleys and have checked out viewpoints I never would have before. It's also a semi-social activity as you interact with your buddies on a delayed basis or via a website but once in a while you can attend a social event where you finally get to meet fellow cachers and swap stories.
This is one of the few out-of-doors family activities out there so I think it's great on that basis alone. The fact that you have to use your imagination and intuition to find the actual cache is bonus - it keeps the "grey matter" upstairs working! Caching is rarely boring - we have a number of active local cachers that combine games, puzzles, multi-caches, virtual caches, the various features of your gps, and complex slight-of-hand to hide their locations.