Had the product for a couple of weeks half installed and now fully installed.
There were a couple of things people need to be aware of. First is it is a 6" core rather than the 4" core my old extractor in the bathroom was. The second is it needs to have about 12" of wall behind it rather than the double thickness brick wall we had in the Victorian house. As the whole thing needs to be in the wall our electrician had to get a suitable box to put behind it and insulate it so that it behaved properly.
The unit is on all the time and has a very faint draft from it when idling. This brings in fresh air from outside, and apart from the movement of air cooling effect seems similar temperature to inside. However once correctly adjusted by the handy dial the unit powers up fully when the air gets moist. This has more of a movement draught but the air doesn't feel cold and indeed the room stayed a similar temperature throughout using the bath.
Works as expected - am looking forward to having a non icy bathroom in winter.
Criticisms of the unit - it could do with an outside humidity detector as well as an inside one so it doesn't spin up when it is humid inside because it is also humid outside.
Well worth it but be aware fitting it is a full day's job for an electrician.