Just been reading some info about 3 and their mobile broadband. They launched a while ago in Ireland and all was great to start with but then their network got oversubscribed and crippled.
Hopefully the same thing won't happen on their UK network, but who knows. Maybe something to think about if you are going to sign up for an 18month contract with them!
The info I got was from here:
http://www.talk3g.co.uk/showthread.php?t=4417