Actually SIMs aren't free, they're normally $10. You can sometimes find them for $1 if you look around hard enough [1].

I've been using this plan for about a month or so and coverage has indeed been a little spotty in my area but it's good enough for my needs. If you do want to try it out here's a tip port your number: do NOT do select the option to port during the online activation process. Sign up and activate your SIM FIRST and then call in to initiate a number port request. Once you've activated and signed up for the $30 plan with a port request, there's no way to get a temporary number while waiting for the port request to complete.

[1] http://slickdeals.net/f/6332098-tmobile-99cent-sim-card-sale...