The need for a dark room to sleep in drove me to this product. Blackout curtains leak light around the sides and floor, while sticky dark window films leave nasty residues if you have to remove them - and can't be replaced if you temporarily need light.
Having spotted this stuff as a spin-off from a TV "Dragon's Den" invention, I gave it a go and am now on my second purchase. There was nothing wrong with the first - it stuck to the windows just fine for a whole year... but I learned a lot first time round and wanted a second chance to get it right.
As other readers say, there are problems. First, it really does NOT like shaping itself to the window's edge. It won't stick to curved frames, but if you can't overlap the frames, you don't get total blackout as light leaks around the gaps. You have to cut the product very wide and hope it'll stick.
Second, it won't stick if you open a window and the wind gets under the sheet's edge. For both problems, I've found either a stick of "Post It Note Glue" (like a Pritt stick, but with "Post It Note" glue not permanent stuff) or masking tape is needed to secure the edges.
Third, the perforations on the roll of blackouts leak light if you need more than one sheet per window. You have to cover these perforation rows with more blackout to get the proper effect.
Fourth, small pieces of material don't stick at all. Anything less than around 20cm square, I found. Again glue / tape is the answer.
Expensive, and not the miracle it promises to be, but so far the best solution I can come up with.