I've been a Cingular customer for nearly five years now (I was with Cingular when they were known as Pac Bell Wireless!) and I have had excellent service with them: both coverage and customer care. I know it's annoying not to have access to 611 on a 24/7 basis but I have become accustomed to it. (I hear that there is a number that you can reach Cingular Customer Care at on weekends but don't know what it is!)
Granted each situation is different and so I am not about to pass judgement on whether in a given case the right thing was done without knowing all the facts. At every step of my cell service, I have "done my homework" to determine the best wireless choice for me and each time it was AT&T/Cingular. Nobody else offers rollover minutes for those months that you just can't help going over (esp true on a shared plan with two teens) and a 30-day no hassle return policy.
Comparing Verizon to Cingular, you will note that at each price point Cingular gives you more minutes than Verizon (generally Verizon is the lowest among nationwide carriers). I don't know what you mean by "charge for some messages left on the phone." AFAIK, Cingular does not charge you for keeping voicemail messages for an extended period of time (they are deleted after 14 days), and text messages are billed based only on sending or receiving (10 cents each way; 100 messages/month available for 2.99/mo) -- not based on how long they are kept on the phone, that is determined by the phone's settings and capacity itself. Data speeds are slightly faster on Cingular's network, especially if you are in an EDGE market (such as the San Francisco Bay Area, as I am).
Another advantage of switching to Cingular is that you can keep the same phone you had with T-Mobile. Before switching, ask a T-Mobile rep for the unlock code (they will give these out provided you have been a customer for 90 days, and I see that you obviously have been). You can take this code and your phone over to the Cingular store and they will gladly activate it for you.