I bought one when i first got my setup. let me say this, less than a week later i bought a Helix and a Johnson thermostat. if you are low on cash and need a quick fix, the reptitherm 500 is the best of rheostats but i strongly suggest to get a thermostat in the near future.

the reptitherm has a dial that turns all the way from low to high. place the probe in and you have to toy with it for a little while.