Rena Inline Heaters

does anybody have any thorts about  these Rena in line heaters.The tank i recently purchased came with two of them,at first i thort they attached to the water pipe returning to the tank,althow now i have discovered its actualy the other way around and it heats the water as it removes it from the tank but surely this means that as the water travels threw the pipes and down to the external filter and back up again it is cooling down or is this just me over thinking things.i have two filters connected up one is a eheim professional which has its own built in heater so this one shouldnt be a problem but the other one is aqua one 750 which has no heater built in to the filter.