I was on O2 for my 3G, hated the network then, had really poor signal barely got above GPRS for data, shocking!
Moved to Orange and it was a lot better, most of the time get 3G speeds and now with the merger with T-mobile get 3G at decent speed most of the time, Orange CS i haven't required to use so far since iphone 4 launch so don't know what they're like
I will NEVER go to three as there is no customer service. I took out a 30 day rolling contract on a mifi, took 6 months to terminate my contract with them because the call centre staff would lie or refuse to deal with me, ended up cancelling the direct debit and waited for them to contact me, still ended up having to pay a further two months to get them to go away this included going into a three store to get the manager to help, it was appalling, i even had 1 CS rep say they'd get sacked if they followed up with the disconnection and they wouldn't do that.