I used one as my primary device for 2 years. It worked fine. All I needed was voice, sms, a browser, mail reader and a modem (wifi tethering). I had bought a dev device for $75.

Somewhere in the FF OS 2.x series, an upgrade seriously made the keyboard unusable, and I never knew where to turn for bug trackers / support. I had also tried to make IPv6 work, experimented a bit with the app system.. I'm an average programmer, but I participate in a few large projects.. Firefox has always been a mystery to me, with lots of code names and hard to use bug tracker.

Huge fan of Mozilla otherwise, so hurts a bit to say this.. other than that, it was a functional phone for 2 years.