I have three of these, two are in tanks and one is on the shelf because it started acting up and I won'r risk an unreliable heater. I also have a second heater in both tanks with one of these, and so far (2-3 years) the two in the tanks seem to be OK.
When I need new heaters now, I get the Eheim Jager.
BTW, the problems you have experienced with heaters not keeping the tank temperature...do you mean the temperature fluctuates when the heater is set and left alone? Or is the tank temperature higher or lower than what you set on the heater? The latter is comon, as so long as the temperature remains stable, the setting may have to be lower/higher. For example, my Eheim keeps the tank at 77-78F but it is set on 74F. I've had several different heaters like this.
Byron.