I've been running a HTC One S on Talk Mobile for the past 9 months. They're a virtual network (powered by Vodaphone) owned by Carphone Warehouse. I got a great deal - 200mins, 500 texts and unlimited data for £16.50 a month and the phone was "free", but today had a problem with the network (texts were failing to send and calls were dropping out).
I called their help line and they were absolutely super, and rectified my issue within 2 mins. Really pleasant, knowlegeable and polite customer service. Hats of to them!