I've been with Virgin for years, on a very good SIM only contract, which is supposed to be fixed price for as long as I go on paying the bill.
They use T-Mobile's transmission network (originally one to one), so coverage is T-mobile's, and is patchy in some areas of the country. Irrelevant once the Orange and T-mobile networks are merged with a promise of best coverage of all networks. I've never used it, but I read that the 3G service for the combined network will be the present Orange 3G.
I've always found the customer service very good when needed. That's been when the SIM card has been damaged by getting wet and needs replacing (along with the phone, which is just the cheapest I can get).
I must be odd - I can't get exited about phones (or cars). I'm quite happy with the most basic phone, cheapest is best. Voice, text, and a 200 entry number store is all I want. The one I'm using now was "last year's model", being flogged off on the last day of the Argos sale in summer 2006. £7.50 including VAT.