FWIW I build many of these smaller mobile phone companies (what are referred to as MVNOs). As long as you have your own phone (and it's the correct phone for the network to take full advantage of the network's data features), and you don't leave the US (in the case of TMOB), you can get some pretty killer deals with some of these companies (TMOB will not allow MVNOs to do international roaming, so if you leave the US, you'd need to have a carrier unlocked phone and buy a local SIM card... dunno about the other MNOs). Anyway, the MVNOs I work with tend to offer plans like $39 for unlimited voice, SMS with 1-2GB of data. T-Mobile is the only company that will wholesale unlimited data to these MVNOs, so many are starting to jump ship from AT&T (which after losing their shirt on a couple of MVNOs has decided they do not like MVNOs much) and Verizon and Sprint.
Personally I use Ultra Mobile for my work phone and will likely migrate my family phones over once I'm out of all my AT&T contracts. (Disclaimer: I was the PM responsible for building the technical platform that integrates Ultra with TMOB)
You can get a list of all MVNOs here:
http://en.wikipedia.org/wiki/List_of_United_States_mobile_virtual_network_operators
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TMOB has been refactoring towers all over the US to use GSM on 1900MHz (ie, 3G compatible with AT&T) to allow 3G use of iPhones on the TMOB network. TMOB has not yet started an LTE rollout, but their 3G speeds are still extremely quick. TMOB does not announce which towers have been converted, but you can check using this user updated map:
http://www.airportal.de/