I have a good friend who's way into geocaching. I've been on three or four expeditions with him. He loves it, but I didn't really enjoy it. For one thing, having the GPS takes the fun out of the orienteering part for me. For that I prefer letterboxing. But some other things I didn't like about it: a cache can be anything, from a PVC pipe to a fusty ziploc bag to a rusty coffee can. We've ended up picking up a lot of nasty trash and stuff on the off-chance it might be a cache. Then too, when we have gotten into the caches, there hasn't been much that was fun about them. A soggy notebook, and a collection of plastic bits and junk meant to show that someone's been there. My friend really enjoys it and claims to have had some great experiences that brought him to nifty satisfying results in beautiful places, but I haven't been with him on any experiences that were enjoyable like that.

I geocache off and on and enjoy it. I won't say I have had transcendental experiences but I enjoy being led to remote spots, even in my hometown, that I may not have known about. The cache itself is almost a secondary part of it, though finding it is fun.
1) Always make sure to bring along a poking stick, helps a lot in determining where it's safe to put your hands.
2) Most geocaching pages will let you know what kind of container you are looking for
3) I generally prefer the easy to medium difficulty finds (terrain is a different matter)
4) easiest time of year to find is early/mid spring, and later fall, before the snow
5) It's a good excuse to get out of the house
6) As long as the expectations aren't too high it's a good group activity, or something to do to kill some time. I went to a wedding a few years back in ME, the day before the wedding the bride was having a small meltdown and needed some alone time, so I hauled out my GPS looked at the website and grabbed everyone who was staying at their place and we had a nifty afternoon, while she de-stressed at home.

I suppose I would need a GPS unit to know what geocaching is.

I brought my GPS with me when I went hiking last weekend and recorded the location of a couple of caches along my route. Both of them were off the trail, one of which was down a pretty steep slope full of rocks and pointy things. I never found either of them because I felt bad about leaving the trail and doing damage to the area.

A tree also attacked me as I moved it away to climb around it. I took that as a sign.

I've done several projects to develop GPS-enabled tours through local historic parks. The idea being to use the GPS and waypoints to guide tourists from point to point. In one, we had costumed interpreters at each stop to explain what was there. More recently, we worked up a GPS scavenger hunt in a park that encompasses the historic parts of our state capital. We created clues at each stop that players were to decipher and solve. It was a good way to get people to check out small but interesting historic spots scattered around the town.

It was fun.

clues at each stop that players were to decipher and solve

That would be more fun for me than plain geocaching, where few hunts are all that hard and it's all about just being sharp-eyed enough to find the cache while letting the GPS do most of the work.

There are some people that write geocache clues that way. We tried one last summer that stymied us - never found the cache, although we learned a lot of history of the town in the course of the hunt. It was something to do with a shipwreck.