I was with O2 for 7 years and then gave in to a marketing call from 3.
I have been with 3 for a few months now and have had no problems what-so-ever and their customer service team are a damn sight more polite than the O2 lot!
With O2 i was getting 400 mins and 50 texts for 40 quid a month
With 3 i got a new phone , 750 mins to any network and 150 texts for 20 quid a month
My wife got a 3 phone about 12 months ago and the coverage was a bit iffy then but she has had no problems in that respect for a long time hence me getting one!