From what I have seen, they don't really block lots of light, as in your room gets dark. What they do is become frosted, which gives you privacy.
My thought was to use this on a large bedroom window we have. We have installed automated drapes, which work great, and completely blackout the light, but my wife doesn't like it when she takes a shower because she thinks people can see in. (They really can't) So the only solution is to close the drapes and its dark, or open them and people can see in, supposedly. If we could frost the window, that would solve the problem. Also we get lots of sun in this window, so at times if we could block some of this light a bit, that would be good.
If you have a bedroom, where it needs to be dark, I don't think this would work as your only covering. With drapes or blinds, it would work.