My Hydor Theo just almost cooked my poor betta, so I was searching the board for heater recommendations when I came across this thread.
I've had the Hydor Theo 25W for a little over a year. I retired it to hospital tank duty when I upgraded my betta to a 5 gallon tank. Since my betta is currently fighting some fin rot, I set him up with his old heater. Now, I'm lucky he isn't already dead because of it.
I checked on him before bed and found his tank at 92 degrees! What's really bad is that the heater was set as far down as it goes, 2 dots below 71. I couldn't set it to a temperature anymore because over the course of the year I've had it, the temperature kept drifting upwards. I figured it was just a calibration issue, but now apparently it just stays on all the time no matter what the water temperature. Even after I unplugged it for a few minutes, it came on as soon as I plugged it back in!
It's now going in the trash. Hopefully my betta won't be following it! :( I remember reading Amazon reviews where some people had similar problems with it, and sure enough when I went to look for heaters just now, I came across similar cautionary tales. I wish I had not ignored them when I originally bought my heater, but at the time it seemed worse to not have a heater I could set.
I have an Eheim Jager aquarium heater in my betta's main tank, and it has been rock-solid for the 6 months I've had it, plus it can be manually calibrated.
In my big fish tank, I've had a 300W Marineland VisiTherm that's been chugging along for about 10 years now... I plan on replacing it with an Eheim Jager soon, though, since it's getting pretty worn out.
Unfortunately, the smallest Eheim Jager heater is the 25W, and it still quite large, too big for my 1.5g hospital tank... it just barely fits vertically in my 5 gallon.
I would seriously caution anyone about getting a Hydor Theo, or to keep a very close eye on it if you already use one.