I do it for a couple reasons. One reason is that it's actually able to get me outside, and I go to places that I ordinarily never would have considered going. At my university, for example, I've found several caches located at monuments that I ordinarily never would have gone to (well, maybe I would have gone eventually, but caching gets me there quicker). I also do it because the thrill of the hunt provides a nice escape from schoolwork and life in general. And its fun, but I'm sure you all knew that already.
My university actually offers a Freshman seminar course in GPS technology, with geocaching as part of the course. I wasn't able to take it, unfortunately.