The Ranco and Johnson work great. I've used both. Now, since I have a Herpstat Pro, I use the Ranco for regulating my herp room space heater and I keep the Johnson as a spare. They will keep your temps at whatever you set them at. They do have a differential setting, which is the range above and below the set temperature that they will operate at. So if you set the temp at 91F and the differential is set to 1, that means the t-stat will be on until the temp reaches 92F then it will shut off until the temp reaches 90F. They do make a click sound when they turn on/off. Some people don't like this, but it never bothered me. About the only other thermostat that you might be interested in that some people use is the Hydrofarm...
http://www.urbangardensolutions.com/...tm?click=11398
http://www.hydroasis.com/hy/productdetail.aspx?id=375
http://www.amazon.com/Hydrofarm-MTPR...1529683&sr=1-7