I am torn at the moment. I got sick of Verizon's cost inflating month by month, and wound up wanting a Nexus 4, which led me to T-Mobile. I went with the little-known (at the time) $30/month prepaid plan, with 5GB data, unlimited text and only 100 minutes of voice. I have to agree with
@mountainbikermark that having no coverage out in rural areas is not so comforting. I'm not worried about 911--I still have my HTC Thunderbolt, and can plug it in to make a free 911 call if there is an emergency. But I took a trip to the east coast this past summer and used navigation the entire way. I had to keep my Nexus 7 continually powered on and running so I did not lose Google Nav. Before the trip, I saved maps all along the way, but while the maps did help and navigation worked OK, I could not zoom in if needed (not unless I zoomed in when I saved the map). And there are still cities where TMo has no coverage, like Bar Harbor, Maine. In some cases I had limited phone service through other networks, but data was tough.
100 minutes isn't an issue though. I've had decent luck calling using Google Hangouts. It also lets me access my additional Google Voice numbers. At home I have a good WiFi connection, but I've used a Hangouts voice call over TMo's data network when I have a good signal, and it works well enough to carry on a conversation most of the time. I do get some dead spots indoors, in places where my daughter's phone gets a strong signal.
So while I like the prepaid plan, the price, TMo's customer service, etc., that lack of signal is bothersome at times. The other Nexus 4 in the house is also getting very strange (shuts itself down far too often), and it drops calls. So, a phone and carrier change may be in order.