I am a former Cingular user and present Sprint user. I travel to Europe and Asia a lot, so I am tempted to switch to a GSM carrier. The reasons that I don't:
1) Sprint has excellent service in most of the places where I work and live (SF Bay Area)
2)Sprint has excellent coverage in the major metro areas of the US where I travel
3)I actually think CDMA technology is better than GSM and that Sprint has pretty cool phones. The Sanyo and Samsung phones are pretty good and check out some of the stuff being sold in China for CDMA
4)I have a great retention plan where I get all the minutes I need for 50/month
5)While I have heard horror stories about Sprint cust service, I have always had good luck. If I do get someone that is not helping me, I just hang up and try again. I have also played the "Cancel service" card a few times. This usually gets you to a higher level of agent.

If I were shopping for a carrier, the main thing I would consider is signal strength in the places where I use my phone. If you are considering Cingular, see if you can borrow someone's phone for an hour or more and test it out in your commute, in your house and office (or wherever you use it most). Also, talk to people in your office or neighborhood.

I firmly believe that all of the carriers "suck" in one way or another (price/service/signal), so you have to find the one that "sucks" the least for you and your needs. Hope this helps.

FDuvall