I am very happy with my Hi Power, but with my floor mounted 1272 and Hi Power screen there is a very SMALL sweet spot. The reason I am happy with that is because the worst that can happen is viewers not in the sweet spot (and it's only big enough for one) see a normal image like matt white provides, and usefully brighter if there are only a couple of them.
The gain drops off rapidly when you move either vertically or horizontally away from directly behind the projector. If I sit dead centre, low and behind the projector, then the people sitting immediately to my left and right on the couch will be watching a different image 'intensity'. It will be noticeably down on the sweet spot but still a useful improvement on matt white. Anyone to their side would be pretty much be looking at a matt white type image.
IMO this screen is definitely not for ceiling mounts unless the projector is a foot and a half above your head when seated (or lower).