I put some on a South facing round window and it does cut down on the heat
coming in but I've been told the it is not good for double glazed windows as
it creates too much heat between the glass and shortens the life of the
seal. (It's been two years now and I don't have condensation between the
glass but time will tell.

When I had a problem with the heat and a sky light, I made a solar screen to
fit below the skylight for the hot months and removed it for the winter...
It did cut down on the heat in the Summer and we benefited from the heat in
the winter.