I recently ported from VZW to T-Mobile. It really depends on the area where you live. Some providers work better in some. You should try looking at Consumer Reports. Another thing to consider is the type of network, CDMA or GSM.

My reasons for switching are because I plan on doing a lot of outside traveling and I prefer the freedom of the SIM card that the GSM network provides. I was with VZW for about 3 years and they have the best CDMA network, along with great customer service. T-Mobile had a better plan to suit my calling habits than ATT and Cingular. There's a lot of variety out there, you just have to know exactly what you want and see which carrier suits you.

My best friend has T-Mobile and shes been with them since they were Voicestream. She's never had a problem and is very pleased with their service. I've had too many local people complain about ATT Wireless. So like I said, its all about the area you live in and your calling habits/plans that would be sufficient.

Hope that helps!