The material is light, but very strong and certainly waterproof. It's been raining non-stop for two weeks, and everything is nice and dry underneath. The only thing I don't like about it is that is doesn't fit my bicycle quite so neatly as the one in the picture. Granted, every bike is different. I have a city bike with wide handlebars, so the cover doesn't drape so well. Still, it's only cosmetic as the cover works fine.
One other note. This model does not have any through-holes for a lock. You'll need a good length of chain to lock the bike with the cover fully in place. I believe they make an Urban model that includes a lock through-hole, although they seem to want an additional £10 for the trouble.