I have the BMW half cover, it was perfect for protecting my cars roof over the summer, as birds and tree sap are a big problem where it's parked. Now the winter is here, I don't really use it since the roof is almost always wet when I park up and the birds and tree sap aren't a problem any more.
It does a goof job of protecting the roof without having much contact with the cars body to scuff the paint. The roof does still get plenty of air to it, from the edges, it probably does more harm to the cover if you put it on when the roof is wet, as it will just get dirty on the underside.