I used to sell them, there are a lot of myths surrounding what they can and can't do and what they should be used for.
The main issue with them is pressure drop through the ductwork. Generally they are small units, sized to fit above a domestic boiler, but some can be loft installed. The downside to this is the fans are generally low power due to their size.
Next issue is ductwork through a house, to fit in wall spaces and ceiling spaces it has to be very narrow and a lot of bends, this adds resistance and causes pressure drop. Next add your pressure drops across filters, heating elements, Heat exchangers and outlets and the air flow of many of these units drop to zero.
I've been in some properties when they have been installed well, but generally the house has to be built to accommodate them, or, as above, be in a building like a barn where running ductwork is not an issue.
The best houses i've seen them retrofitted to is bungalows where all the ductwork can be run in the roof space.
They can make a massive difference to heating bills and a well ventilated property is a lovely thing to have. However I can't stress how much these need to be designed properly.
If anybody needs any more info please pm me.