I dont use one for incubators just my racks, and they work well....you need a good quality TStat, I made an icubator once out of an aquarium heater, water in the bottom and seperate tubs to keep the eggs in. Believe it or not they were all parts I had in the garage that werent being used.

Now I use a freezer with FlexWatt a Helix, etc....your typical DIY bator.