In my town/village which is mostly in the country. o2 has really bad coverage. However since i've switched to Virgin Mobile i fin wherever i am i can always get a signal.
Where I live (near NYC, sorta) TMobile was awful. I switched to Cingular years ago, but they just got bought by AT&T, whose service I hate. I'll wait for an opportunity to get out of the contract now.
Im from the Uk, at my place 3 is the worst network, oh and guess what network i have? *doh* I live in a valley though, so no network is good, just i happen to have the worst.