I have T-Mobile and will probably be leaving it. To their credit, T-Mobile offers incredibly detailed neighborhood maps showing tower strength in your area (much better than the general coverage maps of other carriers). You can find it buried in their website. In addition, they also have a policy of 'unlocking' your phone after 6 months so in case you travel overseas you can use the same phone simply by swapping in a local SIM card (which is really convenient and cheaper).
But their coverage is spotty. In Calvert, they have a dead zone on the hills just approaching Chesapeake Beach, Chesapeake Beach -> Plum Point Road, Rt 4 around Lusby, Rt. 4 after you cross the bridge over the Patuxent. On the way to DC it goes out in Upper Marlboro, and one or two curves of the Suitland Parkway, along with by the Jefferson Memorial. The list goes on and on....
I'm probably switching to Verizon, in large part because of the negative comments I've seen here about Cingular. I think Verizon is the anti-christ, but I can't have my commute to DC interrupted four times due to dead zones...