The plans are great, but the coverage area still needs a lot of work. I put my old iPhone 4s on the $30 plan to give it a try as I am traveling this month. I've been really considering switching. So far I'm been disappointed in the coverage. Just too many areas where I got "No Service", not even Edge. Same spot my iPhone 5s on AT&T I get coverage.
It is pretty disappointing when you don't have service on a major interstate (Like I-10 between Phoenix and LA)
I don't know how the 4S radios compare to the 3GS, so what I'm about to say may not be completely relevant. But...
I had an iPhone 3GS on T-Mobile's network, and it only got Edge most of the time - including at my home. But when I upgraded to an iPhone 5, suddenly all those "Edge" locations became LTE locations! So the new bands in the more recent iPhones might make a difference there.
(We still have the 3GS, so it's easy to compare - thought I'd mention that in case someone wondered if the differences we saw were due to T-Mobile's rather aggressive network upgrades of late. It's not.)