i just turned on a ventilator, in the high summer, middle august. so since april i think. like there is some airflow, the air temperature now is around 14 degrees, but no fan and no reported error (actually less than 1% or something like that) on bfgminer and i never had a problem.. ROI is hard to calculate, specially if you think on the price when it came into the market, but for sure they are headache zero and silent.  I prefer a 10GHs from erupter than the asic blaster (I have one, and I always think that it will burn my house down, that sometimes I have to reboot, etc)