It's okay if you have no other choice. Moved into a rented house in December and will be moving abroad in April, so didn't want a 12 month contract.
I'm on 3 as well and it's been fine. As said above, as long as you aren't a gamer, it'll be fine if the signal is strong enough and 3GB a month for £15 is all you'll need as long as you aren't downloading films and the like.