Letterboxing isn't always about the "hunt" for the hiding place, a lot of the times it is just about being outside, going to a new place that you haven't gone to before. And when you find the box, the stamp is the real prize, seeing the craftsmanship (or even lack there of) is a huge part of the fun. We really enjoy looking through the logbooks at all of the handcarved stamps, seeing where people come from and reading the little notes that people write - I have read some hilarious anecdotes about what people have gone through to find that box. Sometimes you'll luck out and find directions or clues to another nearby box that wasn't listed on the site.
And just to clarify, I'm not knocking geocaching, i really enjoy the sport. I'm just pointing out some differences.
