I am a very happy AT&T Wireless customers(errr Cingular customer) as where I live in the Metro NYC area all cell phone providers are pretty good with few dead spots. Plus with my Corp discount roaming INTL is really cheap and sometimes cheaper than buying a pre-paid card for the country that I am in.

Because I travel to Europe and Asia a GSM phone is a must and that rules out Sprint/Nextel and Verizon which are the other larger providers. I used to use Verizon but I fed up with their very expensive service and lower quality calls.
However, I have to give credit to Verizon for their EV-DO service that they realeased here in the NYC/Metro area, it has realy improved the train ride from NY PENN to Washington, DC.

A good friend of mine uses Sprint and their calls seem to be clearer than Verizon's but, I do not like their handset lineup.

Dan