have been with
Vodafone (expensive, customer service crap, will never use them again)
O2 (VERY Crap customer service, didnt get the impression anyone knew what they were doing, plus they sponsor the scum)
Orange (Great Customer Service, although handset range isnt the widest, and you dont get that good upgrade deals)
3 - no probs with service of the network, although having a non-UK call centre is a pain in the arse, esp if you've got a really complex problem to explain. good that a board member is high up the chain to sort my probs out!
t-mobile - never been with em, although offer of a free PSP, LCD TV, X-BOX 360 looks mighty tempting...