i am with 3 for 9 months now.
What I like about 3:
Good value service
I can use my inclusive units to use my mobile as a modem for my laptop without breaking the bank
Simple tariff structure
What I dont like about 3:
call centre staff can take a while to get the message!
signal can drop to zero for no reason ( even in the cities)
No iPhone ( i love my gadgets)
Will I stay with 3? I will as my bills are lower than with O2 or vodafone, and it is really easy to find out if you have exceeded your allowance. But they could do better