I have used Orange, O2 (when it was Cellnet), and One2One/T-Mobile - and more recently Virgin Mobile. Technically, I found Orange to be the best network, O2 at the time where I live was un-usable (only had the contract a few weeks), and was with One2One/T-Mobile quite some time. I was very happy with Virgin Mobile - very good value for money, and excellent customer service (by far the best I have recevied) - only occasional technical issues. Free voicemail is also essential - I don't think networks should be allowed to charge for this, or itemised billing like Orange are doing now. However, now Virgin Mobile have merged with NTL, I am seriously considering changing again, probably back to Orange Pay-As-You-Go.
Steve.