We've had Alltel for 6 years now and had never had any problems with them. I find their customer service outstanding and quick to respond to any questions.
Also after Katrina happened they gave us two months of unlimited roaming, long distance, SMS messages etc. - we kept paying our regular rate and were not charged any overages. The CS rep actually told me, "We WANT you to call everybody and talk to them and tell them you are safe." The number of calls we've made, especially with my husband not having any roaming included in his plan, would have probably been well over $1000+. As far as I am concerned, they are an absolute #1 and it would take something really monumental for me to switch to anybody else. They are also really nice if you have any issues with your actual phone.