We have one - well a different brand but same idea. Ours seems to be very efficient. We have one room that does not have a heat duct so it is always cooler that the rest of the house. It is my sewing room, so the heater is in there. We have a heat pump for the rest of the house, so that is efficient to begin with, so I can't tell you how much we save $$ wise. My husband saves his sanity because I no longer gripe/whine/yell/beg him to make my room warn. Since he pays the bills, I don't know much, but I'm sure I've heard him say the bill didn't go up much.
Ours, for those who might want to know, is a Sunheat
http://www.sunheat.com/howitworkstab.html