Like MNWayne, I monitor the fermentation temperature, but the probe of my controller is in air to control ambient. (I've seen manufacturers recommend this.) The large thermal mass of the liquid relative to air keeps it steady while the air temperature fluctuates around the set point. While some people have luck with the probe in a thermowell or taped to the fermenter, I'm one who hasn't. When I tried the probe in a thermowell, either heating or cooling was running nonstop, driving ambient too far in one direction or the other, and the beer, due to its thermal mass being slower to respond, would eventually catch up and thus had wild fluctuations in temperature. Controlling ambient is easier on the equipment and more successful in maintaining even temperature in the ferment. I just have to watch the temperature of the ferment, because the differential between it and ambient will vary with the vigor of fermentation. Adjust as necessary.
Oh, and since I switched to fermenting in a 10 gallon corny last year and no longer have a thermowell, I've gone old school for monitoring the fermenter temperature. Stick-on liquid crystal Fermometer.