Been a happy AT&T customer for 10 years now. We had Sprint before and they were decent back in the day, but I think smartphones and poor leadership really killed their network in the recent past.
Verizon's network is pretty good, but they're equal in my market and so, there's no point for me to pay extra for them. Also, I'm not a fan of CDMA, although, I have to admit that many of the qualms that I had with Verizon are almost a non issue now, such as the lack of GSM roaming, switching phones being a pain, slow internet, etc. Simultaneous voice and data is still missing on the iPhone, but a lot of Androids have it and it works fine.
Still though, I'd rather fall back to HSPA/+ than CDMA 3G if I were not in an LTE zone. I get 8-12Mb/s without LTE, so I'm pretty happy with that and my bill, which would cost at least $60 more on Verizon.