I bought one way back when I first got my bp, and I have to say I really don't like it. Even when I had it turned down as far as it would go, it only lowered the UTH from 112 to around 100, at least according to my digital thermometer. I've found it works better with heat bulbs, and only if your room temps don't fluctuate too much (as was already mentioned).

I would also highly recommend the Hydrofarm thermostat. Amazing quality for the price Or a cheap lamp dimmer from Home Depot or Lowes is pretty much the same as the zoo med rheostat, and is a lot cheaper.