Having worked my through all of the main providers in the UK over the past few years, I have to say for me O2 win hands down. Orange has zero coverage in my area, despite them saying opposite, all of the others were just OK. O2 almost always have full reception here, although 3G does not work anywhere in my home town for some reason so I miss out on Video calls.
O2 are very open to negotiations, I have 2 phones on contract, both 12 months, and after my initial contract expired I asked them what they could do for me. Aftr some haggling I got on phone 1 500 x-net anytime minutes and 1000 texts for 25 quid per month. Second handset, the one I use, I negotiated 450 x-net anytime minutes, 750 texts and there unlimited data plan, 3 gig monthly fair usage, for 25 quid too.
The only issue I ever had was ordering my
N95, they said in stock and guaranteed next day, no phone after 4 days so I rang them, again guaranteed next day, 2 more days pass. They finally admitted the ohone was never in stock to start with and they were simply taking pre-orders. Complained and got a £50 credit on my account.
Overall they have been great to deal with.
Absolute worst was Orange, guaranteed me top notch coverage, actually get no signal anywhere in my street. If I walk around the local area I could sometimes get a signal. They refused to let me out of the contract without penalty. In the end I just cancelled the direct debit and returne the phone. This was 3 years ago and they are still trying to get the remaining line rental costs for breach of contract LOL