We had tons of issues with AT&T. Incorrect billing, getting random charges that weren't ours, and a huge pile of "you have full bars, but absolutely no data and calls can't go through" issues. (during the early iPhone days I worked at the Apple Store, we had a standing policy in several local stores of sending one of our sales reps to the AT&T store across the mall with a customer to get issues resolved because AT&T lied to customers so often to try and upsell them to something else, or out of sheer incompetence.)
After switching to T-Mobile we've had great service, no billing issues, no problems with international roaming or surprise charges, and a couple of cold-calls with offers to update to a new plan which was actually cheaper. The only issues we've ever had have turned out to be hardware related.
I think the truth may be that mobile carriers, much like ISP's, are a complete crapshoot, and reliability/service may entirely depend on which market you're in.