Takes up almost no room and there are all kinds of things you can do with it -- explosive ballistic movements like swings, and slow grinds, passing it around the body in hand to hand drills, etc. Strength, endurance, or what they're calling now "strength-endurance," whatever. I really enjoy mine and think it's a terrific value even though it's so expensive.
I got the set of Steve Kotter's FullKOntact Kettlebells DVD's in the package deal when I got mine. Those DVD's are excellent, especially the second one. Some very creative exercises.
Here's a link to a guy who did a few e-book type thingies with a bunch of kettlebell exercises. I bought the encyclopedia of KB exercises. Very cheap, but still pretty good, with lots of exercises:
http://www.girevoysport.ru/
The standard kettlebell reference people refer to is Pavel Tstatsouline's Russian Kettlebell Challenge book. It comes in DVD too, but the book has a lot more info than the DVD does.
I think all of these products are worth buying. Cotter's are my favorite, though, but they are more specialized toward martial arts.