I use them in NYC, and I'm never looking back. I pay $30 a month for unlimited data, unlimited texts and 100 minutes of calls. I bought my phone (Nexus 4) outright for $350. I even tried out hacking LTE to work with it, and I get T-Mobile LTE in Brooklyn (though not in Manhattan).

In summary; it's awesome. However, when I leave the city, coverage is definitely limited. I do so very infrequently, so it's fine for me.