I got the opportunity to buy a T2 Mobile Flame (official Firefox OS developer phone from Mozilla) for cheap (60 €) last week. While it would not be fair to compare this phone to my current daily driver phone (Nexus 5) I have to say that I am mostly pleased with the Flame.
It is still far from replacing Android for me, but it will give me the opportunity to follow Firefox OS development more closely. I am already compiling a list of things that keep me from using a Firefox OS phone instead of a Nexus and will report everything on Bugzilla, hoping that at some point the Firefox OS will mature enough to suit my needs.