We bought a 5er with the useless fire place and had it yanked out and cabinet doors installed over the hole and I added a shelf.
You get the exact-same heat out put ...1500 Watts as the little cube heaters. The cube heaters take up less room and you can move them to what ever area you want heated.
And if the manufacturer sticks the thermostat right over or next to the fireplace...the thermostat is nice and warm and the furnace will never come on if you are operating the fireplace.
Other than the ambiance, they are a giant waste of space and money in my opinion...but that is my opinion.
Ken