I've been with 3 for a month now, well I get my first bill tomorrow, and other than not being able to get a signal at my friends house I've had no problems, except being caught by hidden charges from viewing porn vids online.
I failed to see the "watching this video will cost you £2" when I thought it was a free vid lol. My own fault I guess, so much for £25 a month this month lol.
Unlimited Data does kick ass, especially for tethering since my work PC doesn't let me go on social networking or media streaming sites