On the recommendation of a friend, I recently picked up one of these for use on backpacking and bikepacking trips. It cost $15 at my local outdoor shop, and in it's stuff sack actually weighs exactly 3.8 oz, as advertised.
I used it on an overnight mountain bike trip this past weekend. No, it did not rain, but I live in Arizona and precip usually isn't an issue. Hence this bivvy; tough enough to use as a ground cloth to preserve the thermarest, light enough to take anywhere, and waterproof in the case of an emergency.
The material does indeed seem like it will last a while, and it's big enough that I'm sure my 5'11" self could hide inside well enough to stay dry in a good rain.
My only caveat thus far is the tiny, long cylindrical stuff sack. Once you unroll the bivvy for use, I found it too much of a bother to fit it back in the factory sack. I've substituted a squatter, slightly larger sack to make my life easier.
Light and cheap, what more could you want?
