If you want an all-purpose, all-weather bivy sack for ultralight camping, you should check out Outdoor Research’s Alpine Bivy. This bivy sack is well made, fairly roomy and should hold up to some pretty tough conditions.
While some bivy sacks are nothing more than a waterproof covering for your sleeping back, the Alpine Bivy is almost like a miniature tent. It provides enough space for a tall guy like me (6 foot, 2 inches) to fit in and it includes a pole that holds the upper portion of the bivy up above your face, giving you plenty of breathing room.
Before the Alpine Bivy, I had a fairly simply bivy sack. It provided protection for me when I went camping on trips where I knew it wasn’t going to rain (or at least it was very unlikely). It kept my bag dry and protected it from the ground. However it just wasn’t really cutting it, especially on trips where there was a greater chance for wet weather. That’s why I started to look for different options.