I had a pro-heat go berzerk. Luckily the therm went on it. Imagine walking into your fish room to find the therm reading 102. I freaked. Of course one touch of the water (what was I thinking, lol... imagine if it really was 102) told me that wasn't right. Turns out that it was not 102 but more like 72 since the heater had stopped coming on. I was one of the lucky ones. A lot of times the go the other way and fry the tank.
I have 2 of those and 2 of the Stealths in my system right now and would love an alternative to the Pro-Heat. I've heard lots of problems with them (of course not until I had a problem with mine, lol). I wouldn't run one on a system without a temperature controller as a backup.