i have been using tmobile for what must be almost 3 years, get on well with them, havent needed to call customer services too often except to get answerphone numbers when i updated the software on my phone. when i have used it it seemed good. they have good insurance too.
i like their tariffs, i have the flext + webnwalk max or what ever it is @ £35/month with unlimited internet (i can use my n97 as a modem on my laptop if i need to).
coverage is ok where i am and have been. usually get 3g anywhere but my home town :S