I've had three Hydor Theo heaters. My original trusty 25 watt gave out last week (actually, it started heating out of control, fortunately I noticed when the water was at 90 degrees and the thing was still heating, and Galahad was still alive. I tried fiddling with it, but it seems to have become stuck in the heating mode on any temp. Before that, I had needed to turn it way down for about a week to keep the temp down at 80ish...now I know why. Prior to that, it had been very trusty. My 50 watt (which is in a 3 gallon) works great, I keep it turned just a touch down (it's on ~78, keeps the tank right at 80). The new 25 watt I got for Galahad's 1.75 gallon, which just got upgraded to a 3 gallon, is on about 79 degrees and keeps it right at 81.
They are not perfect, but they are great little heaters. Nice and small, fitting in even tiny aquariums. I find I do need to re-calibrate them depending on the season (so in winter, when my apartment is always quite cool, maybe 60-65 degrees, I turn the heater up to 84 or so to keep things on 80, and when it got hot, I turned them down to the upper 70's to keep things on 80ish. I will agree they like to heat to 82 unless you are really watchful about calibrating juuuuuuust right.