if the external hydor can only raise the temp by 10 deg F above ambient room temp, then it really isn't that great of a heater, is it? it seems to me that the heater should be able to raise it to the temp you want regardless of ambient room temp. i suppose all heaters are affected in some way by room temp tho, since winter time brings temps way down in some areas and requires heaters to work harder to maintain temp. i guess it's lucky for me that i live in the sf bay area, where temps are generally mild, even in winter.
still, consensus is in favor, so must be a pretty good design.