In the Northeast, Verizon rules, as it does in other parts of the country. No other carrier seems to be able to match their signal strength. Truth be told, I haven't had a chance to test the new Cingular since AT&T has been rolled in.
Speaking of GSM networks, when I switched to a GSM network a while back I was amazed that there was no analog roaming support built-in so a dead-zone was a real dead-zone. Now that I am using Verizon again, I am able to connect to an analog signal even when digital is not available. This makes life a great deal better for me as it gives me the option of being connected even more often. Analog calling eats more battery but sometimes you just need to have access and battery life is less important.
I was thinking about this today when I read an article about how consumers in rural areas are concerned that analog towers are going to be phased out in 2008. Obviously there are emergency notification issues with the phasing out of analog signals. Here is the full article titled Residents Fight to Keep Analog Cell Phones.