I use them. It was the best deal I could find to entice my sister and brother-in-law to get cell phones, because it is only $6/month per extra line.
I have generally liked them. The plans are reasonably priced.
However, I think they use Sprint's network, which can be very spotty in areas. For example, I vacationed in a more rural part of the Northeast recently, and my phone was pretty much dead the whole week. The other people I was traveling with had Verizon service, and their coverage was infinitely better than mine.
In my hometown, however, the coverage is great.