I have found a number of makes and models of fans mounted both on top and underneath existing radiators, I have got a Myson fan assisted radiator and it works well, but unlike normal radiators there are no lock shield valves or TRV's the water has free passage and output is controlled by fan speed and switching fan off/on, it also will not activate fan until radiator is warm, main advantage it is very small for the output, and furniture placed in front like chairs or couch does not reduce output.
Having seen the add ons my thought was try a upright fan next to radiator with my beer brewing thermostat so it only turns on when the return pipe is hot, it cycled off/on as it was extracting more than the TRV and lock shield valve would allow to pass.
I have experimented also with geofencing, however again a failure as it takes too long to heat the room mainly due to the anti hysteresis software built into the electronic TRV head.
As to the electronic head, again speed was a problem, if the lock shield valve is open too wide, then the room over heats before the TRV has had chance to close, so the setting of the lock shield valve is very important.
I can see the advantage circulating the warm air and so increase the speed at which a room will heat to and settle at a new temperature, the idea is good, however I can't see how they would be controlled, seems there are three makes, rad booster, 4 your home booster and radfan some on top some underneath and some either/or models. The power of fan varies one states 2 watt other says will work from USB so must be a small fan, they do state things like on at 30°C and off at 28°C but don't seem to measure room temperature as far as I can see, really if room target is 20°C and room is at 14°C then one wants the fan to run, but once the room hits 18°C the TRV will start to close so at that point the fan really needs to turn off.
Ideal would be an IFTTT link between the TRV head and fan, but it seems there is very little control, so it seems can be run in the summer, others not. I know the new iVector Myson can have two matrix's so it can cool rooms on one side of building while heating rooms on the other side, and has auto selecting fan speed depending on gap between target and current temperature, so the add on units are no where near the real thing, but they are a lot cheaper.
So has anyone any thoughts about the add on units, to my mind if the boiler can give 8 kW to 28 kW to heat a room fast likely it has to be that other rooms heat slower unless some one did not do their sums before fitting, but interested to see what others think.