I've seen a few examples.
The dark version is awful, the white looks nice as does the pale yellow.
If you have a room that is overwhelmed by the sun then they work well, they are not going to do a thing to keep heat in though.
If you ever open a window then they are going to get dusty and they are a faff to clean properly (i.e. don't bother with those daft two spronged brushes).
The installation is what lets these down, they build a second frame inside the sill area and fit them to that, this looks a bit tatty and leaves a dead fly trap behind the new frame.
You can just about make it out in that picture.
Actually a simple blind does a better job, or Roman blinds for something a bit nicer, being as the sun comes down at an angle you can shade the top part of the window but still see out.
If you have a room that really does get too hot and opening a window is a problem, then they are a nice idea, otherwise they have a lot of negatives.