Problem with buying your own equipment: I'm in my early 20's, so haven't settled into a permanent residence yet. I move around fairly often; I doubt this is uncommon. Exercise equipment is difficult to move (often by intention!) so buying it is just cause for headache.
I do own one 15 lb sandbell, which is basically just a more versatile medicine ball. I can do enough exercises with it to avoid not being able to move for three days after a game of ultimate. Anything more would be much too inconvenient.
Seconded, they are extremely versatile, but make sure you do some research on how to use them effectively (books, videos, or trainers). Their unconventional shape, which makes them so effective, also makes it easy to lift in a way that is bio-mechanically unsound. Also, there are a huge number of compound lifts that you would never think of on your own (e.g., the Turkish Get Up—not a beginners' move for sure: http://www.youtube.com/watch?v=uGRBvom4Zrw)
I do own one 15 lb sandbell, which is basically just a more versatile medicine ball. I can do enough exercises with it to avoid not being able to move for three days after a game of ultimate. Anything more would be much too inconvenient.