These are very common in planted tanks setups, they are mainly used for aesthetic reasons to keep equipment out of the tank but they also provide more even distribution of heat throughout the tank.
In order for them to work correctly they must be positioned verticially at all times, they will also reduce the flow rate of whatever cannister filter you use with them, although i believe because the heater is tubular in construction the water passes through them and flow rate isnt reduced significently, i havent read any bad reports by people that own them or significient issues with them, i would say overall they have a very good reputation and are very reliable.
