Never had a heater fail on me, then again, I haven't been into fish that long.
Picked up a MasterPet one and didn't realise you weren't suppossed to emerse it. Worked fine on a 28L (55w) but had to crank the temp to 32 to get it to run at 26 :-? . Doesn't work very well anymore, I just use it to heat water change buckets occasionaly.
The trick is to get an appropriate sized heater so it doesn't have to switch on and off heaps.