I've had the opposite experience, was with T-mobile and they where ok, data was a bit **** but my main gripe was calls dropping all the time, this was during the time they merged with EE.
Switched to Three, and life was good, data speeds where insane, unlimited data really changed how I used my phone but throughout 2013 Three went to **** in London. To the point where I had no network almost any time I went indoors. The speeds went from 15mb to 1mb if I was lucky, typically hovering around 300kb.
Now I'm on Vodafone and its amazing, I get 4G coverage all over London, typical 30mb+, Data caps suck but I have 8GB, so as long as I keep an eye on things I'm ok. Contract is coming too an end soon. Might be looking at EE to get a phone on 20GB plan and sell the phone off if it works out to a decent price.