I've been with AT&T (in some form) since 1994. The only time I wasn't, was when work provided me with Nextel and then I did a 3 month stint with Verizon (worst phone service I have ever experienced, as far as performance and customer service).
After that, I went back to Cingular, which went back to AT&T. Mostly for the customer service. I haven't ever had an issue with customer service, in fact it has always been a pleasure to work with them.
My most memorable experience was mid 90s. Someone had cloned my phone number and run up over $800 in calls to Hong Kong. When I called them up, I explained I didn't know anyone in Hong Kong, nor would I have any desire to call anyone in Hong Kong. Without so much as an investigation into the calls, didn't even transfer me to a higher level rep... they wiped the calls from my bill. My next experience with them, was when I actually did run up over $400 in calls. I called and told them, I was going to have to cancel my service and arrange a payment plan, to pay off the balance. Again, without even transferring me to a higher level rep. She asked me what would it take to keep me with them. I told her, I simply didn't have the money and didn't have any other option, but to cancel and make payments. The rep, then said... "What if we were to wipe the bill clean? Would you stay with us?" Well, after I picked my jaw up off the floor, I repeated her offer and said that I would most definitely stay with AT&T if she pulled something off like that. Couple of keystrokes and my balance was $0.
Now, I certainly don't expect them to do something like that again... but it made quite an impression on me... and as I already said, I've always dealt with pleasant people, who provide reasonable solutions and even offers of changing our plan, to save us money.
When I bought my Captivate from an online dealer... $89.99, I called AT&T and asked if they could match the offer. The rep put me on hold, to check with her supervisor. When she came back, she apologized and said she couldn't offer the same price. However, what she did offer me, was to bill me the $199 to our bill, then do a rebate on the bill of $110... making the cost $89. Had I had the money on my debit card at the time, I would have taken her up on the offer.
I think the only beef I have with their service is the 2GB data plan. And that's only in principle. Reality is, I don't come close to using that much data (since I'm usually on WiFi). I could even switch to the lower plan and be safe. I just stick with the 2GB... because, you never know.