Most of the responses failed to mention the most obvious thing about the
Eberspracher they keep you TOASTY WARM.  But they are current pigs for sure.

I've winter camped with one down to -14C plus windchill and it sure made
bedtime more pleasant, taking the chill right out of the air and warming up
the blankets.  It also helps get the frost off your nose in the morning.

Although I've never had the experience of the Propex, I suspect the overall
usefulness is better (able to leave it on all night(?)), but for a pure BTU
rush, the Eberspracher is King.

Fichten