Philly, gr8t! I also travel a lot, so...
In Philadelphia, the burbs and most of Delaware & South & Central Jersey, service is (Stewie) eeexxxxxceeellent! I think service has really improved. Before, there was a dead zone at a spot on the 'blue route (Philly & burbs residents know that route I-476) before joining I-76, you would always tell people about the impending disconnection, not any more on 3G! In NY, downtown is alright, midtown & uptown is okay. Keeps changing from 3G to Edge, which may lead to dropped calls.
Baltimore & DC/Maryland/Virginia burbs, very good. Saw 4 bars only once. Was using Iphone gps, so I know.
Raleigh, and Charlotte NC, very good. Thought it would not be good for some reason.
Boston, Worcester, MA, excellent. Most of Connecticut, Rhode Island (Providence), very good!
However, Dallas, Houston, Austin & San Antonio has probably the most impeccable service. I never saw 4 bars, always 5 bars! Only while driving from either Dallas to Houston or Dallas to San Antonio does the service change from 3G to Edge in between the two cities respectively, but you are in rural Texas!
California, San Fransisco, excellent, LA, okay, Perfect in San Diego!
Las Vegas, 5 bars. I have driven from San Diego to Las Vegas, and the coverage was much better than I expected, no dropped calls whatsoever while crossing the desert!
Chicago, very good, even during the dead of winter when the wind is howling!
Seattle, okay. Most of my pals have T-Mobile there, but I had 4 or 5 bars mostly. Even with 1 bar, no dropped calls.
Kansas City, MO. Not bad. Everybody has Sprint there, no idea why!
I have been with AT&T since it was AT&T, went to Cingular, then to AT&T. Service has improved, cannot really remember the last time I had a dropped call...
AT&T gets a lot of flack, so I think they deserve some praise! Just ask T-Mobile customers in the Bay Area. 0 Bars on Edge in many areas!
Hope this helps!