I purchased one of these rotating cowls 3 years ago because my lounge fire kept puffing huge clouds of smoke into the room every time the wind blew in a certain direction. The problem that I had was downdrafts coming down the chimney due to turbulence over my roof caused by large trees nearby. This cowl fixed the problem instantly. Even in a gale no smoke enters the room.
The unit was very easy to install providing you don't have a problem clambering up onto the roof.
It is however important to understand that this unit is designed to stop downdrafts. I will not necessarily fix a smoking fire caused by other problems such as inadequate ventilation, defective flue, poorly designed fireplace etc.
I burn house coal which makes a lot of soot. The cowl can under certain circumstances soot up. I have found this can happen after a spell of calm weather lasting for 2 or 3 weeks; it would appear that the cowl is normally kept clear of soot by the actions of wind and rain. This is not a problem as I have to sweep my chimney twice during the winter anyway. The action of the brush bumping against the underside of the cowl seems to dislodge most of the soot and the next time it blows or rains completes the cleaning process.
I would imagine that if you are burning wood any creosote tar build ups would be more difficult to remove and might require removal of the cowl for cleaning.
All in all an excellent product providing you understand its limitations.