It worked really well. That Honeywell thing let the temperature cycle over maybe a 2º C range. I was sampling the thermistor 100 times in about 1/10th of a second (analogRead() on Arduino) and then doing exponential averaging each second with a 32 second time constant (i.e. `temp = temp - (temp >> 5) + new_reading`) and averaging and getting a nice stable temperature reading that moved smoothly in 0.01º C increments. Turning the heater on and off at maximum every 30 seconds the room temp usually stayed within ±0.03 C of the set temperature -- not precise on an absolute scale but detecting and correcting small changes accurately.

I was actually thinking of trying to make a product, especially once Flick Electric started up with electricity prices changing every 30 minutes based on the wholesale rate plus a 2c margin. You could (still can) get the current 30 minute wholesale price at your local substation from electricityinfo.co.nz with a simple http query, so you can build something to make intelligent decisions.

But then I got a job overseas and dropped the project...