I just bought one of their SIM cards with their $45 unli everything plan and popped it into my Nokia N9 when I was back in the States for a few weeks recently.
Process was straightforward enough to set it up. I had decent coverage (AT&T, YMMV), unlimited everything was nice to have. Unsure what experience the previous poster had with their 3G speeds. I found it to be quite speedy. Able to watch YouTube, etc. as fast or faster than my connection at home in the Philippines.
Overall, if I were living in the States, I'd surely go with this company as I like my phones unlocked and without the bloatware of the mobile companies.
Even at that, I'll still use them when I return the US so that I have a mobile number where friends and family can reach me, while I'm there.