TMobile coverage in the bay area is not great - I'd definitely stick with AT&T in the bay area. Also, if you get a TMobile Blackberry and it is 3G, the 3G frequencies will be different and not interchangeable with AT&T, even if the phone gets unlocked (and vice versa) due to TMobile's really odd 3G spectrum range. The call quality/dropped call situation is not as good as Verizon Wireless, but definitely far superior to TMobile or Sprint. When reading about AT&T coverage problems, it can be difficult to sort out those that are really AT&T and those that are iPhone problems - but I definitely have fewer problems than my friends with iPhones.