Spent last two years on t-mobile and to be honest couldn't wait for the contract to be over. I'm in south east and the call quality just isn't there on t-mobile, it daleks out on me even in West London, data coverage in car is very patchy - oddly enough it was quite good in sticks of Kent, but they have massive 3G coverage holes in London, with voice and data connection very often non existent or breaking east side of Limehouse Tunnel area of Canary Wharf, south side of Blackwall to Blackheath, The Highway/Embankment around Nomura building to name but few encountered by me every day.
I waited til they merge with Orange, but bizarrely things actually got worse with merge instead of being better. Mobile data in my office location got much slower from end of January and something happened to masts in my home location as I began to see less bars on my phone and started experiencing more battery drain when using phone to browse something in the garden from beginning of March. This is all subjective, I realise that, but I am very happy Tesco (O2) customer for several weeks now and most of my daily in car data gripes have literally vanished with move. Even my stand by battery life is better since I moved away from t-mobile.