This is actually something I'm struggling with right now.
Cingular works beautifully EVERYWHERE in Northern Virgnia. There is only one place where it doesn't work: my office building. As soon as you walk outside, it's five bars. Walk in and you get "No Service".
I've tested all other carriers---all work great, except for the network with the fewest dropped calls.
I'm a Realtor, so my phone is the center of my business. I currently have my phone calls automatically transferred to the office landline phone as soon as my Cingular phone gets no service, but I'm getting increasingly frustrated with this scenario.
I've called Cingular about it---they had me switch phones, SIM cards, etc.---to the point where one representative recommend I switch to another carrier.
And the only reason why I am staying is because I want the iPhone. But therein lies the question: what point is having the greatest phone on Earth if you can't use it to make calls?
It's a tough call.