Unlike some thicker alternatives, this one didn't set my alarm off! It covers a Rav4 windscreen well, being quite large. Don't worry about it being thin as that is not a problem. What is, though is getting it into place. You need to slam both front doors onto it, with it taut across the windscreen. However, as you slam a door, the draught you create blows it away from the screen. However, there doesn't seem to be any way round that apart from having suckers onto the windscreen.
If it is not firm on the screen then you will get ice behind it, but not as much as you would get without it in place: just ridges where it folds away from the screen. My experience in December 2010 has been that it is so cold I get ice on the inside of the windscreen too!
I would give it between 4 and 5 stars overall.