From what I've read on different sites people who built & used them are pleased.
As Torry wrote it won't heat your whole home but hey it's free heat. There's a good 3 month or more portion to the heating season where we could get by with just these two rooms being warm enough to complete our morning routine.
This is the year we're building a few, one each of the bathroom & daughter's bedroom which receive the most direct sunlight.
I figure that it my daughter's can wake up to a warm room and we all can stop dreading the "cold throne", it will be worth it.