I'm in SF and reception is pretty good. My only complaint is dead spots. I've noticed mysterious blackout zones where there should be none ... like around 19th-21st Street and Mission, a major thoroughfare.
But like CaptainIndignant says, cell phone service in SF is a little notorious. T-Mobile probably has the least developed network, but none of them are really great here. Prior to going with T-Mobile (maybe two years ago now) my phone was on Sprint and it truly sucked. My outgoing voicemail message told people to call me at home -- because when I was at home I got zero bars, so the cell was useless to me as a primary phone number. If you were driving in a car, also, you could expect to lose your call as you changed neighborhoods. My T-Mobile BlackBerry 7100t was the first phone that got me consistent enough reception to hand out the number. I think the 8100 might actually have even slightly better reception.
Incidentally, one of the reasons for poor reception in SF is due to SF's typical negative attitude to any form of development, and cell phones in particular. The folks who shop at vegan groceries and go to Burning Man are also generally very negative toward cell phone use (even though they all have them; they just hate to see anyone else with them). They also have very much a "not in my backyard" attitude to development. The result is that, any time one of the major cell phone providers wants to build out extra capacity, the local neighborhood association gets together and lobbies to block it, citing evil radiation/cancer threats etc. The result? Crap cell phone reception. I'm not making this up -- things may have changed more recently, but here's an older article: Resistance Mounts to Cell Phone Antennas