When at Sierra Nevada we saw their recovery system. It turns out the stuff you smell comming out of the fermenter is not what you want to put back in your beer. They remove esters, and if your yeast was kicking out any sulfur you would want to get rid of that. There is also some ethanol in the CO2, which they turned into fuel. For me not worth the bother, CO2 is cheap, and I don't have to brew by the Reinheisgebot.