I was a long long time AT&T victim. Final straw was when I stepped off the train at 2am on a Wednesday morning on the edge of downtown San Jose and listened to a voicemail that I received only 15 minutes earlier. So, I immediately hung up and dialed my friend. I received a "network busy" signal. If you know San Jose.....really?
Since then I have switched to t-mobile and I have never had any similar issue (in three plus years). Only place I have 0-1 bars (and I can make and receive calls clearly) is at home. Pretty much the other parts of San Jose I get 2+. As far as SF is concerned, I get an average of 3 bars. I rarely have issues with signals in SF. But I do notice that when I do, other people on other services that are with me do also. Also, I believe there are parts of SF that just stink for every provider.
As far data is concerned, the max speeds are not amazing at all. But t-mobile has far fewer users (especially in the SF area) than AT&T and so I find my t-mobile data speeds to be consistent regardless of time, day, place, etc. Where as my friends on AT&T, Verizon, etc seem to run into bandwidth congestion issues that I can almost predict for them.
What phone do you have and what plan do you plan on getting? If you have your own unlocked GSM phone you can sign with t-mobile for a month. If it stinks for you and your life, just don't renew and go to another provider. No contract = no risk. (Of course, I am assuming you are not breaking a contract to try them out and have a phone you can use with t-mobile.)