Anyone ever use any of these? See any problems with using them?
Nope,
but as the response curve is not quite linear you might consider using - Arduino Playground - MultiMap - to get a fast lookup.
What will be your operational minimum/maximum?
Its for A small workshop heater. It uses an automotive inline coolant heater A resivoir and A heater core. It keeps the shop surprisingly warm. only problem is I have no way to moniter temps or water levels and at some point would like to plumb in A solar collector that would require control of A circulation pump.Water temp are anywhere from 0C to as high as 90C but operating is around 50C.