I spent quite a bit of time looking at Bivys lately. Planning on a deep Everglades exploration for several days and the thought of spending an unplanned night on my boat due to a motor malfunction or getting lost (turned around ;-) without adequate mosquito protection is a particularly troubling prospect. I have heard of people spending the night in the water rather than face a night out in the Glades without a sanctuary from the winged menace but I digress.
From everything I have read truly lightweight Bivys are not all that much fun. According to one review which tested numerous bivys the mid priced OR (Outdoor Research) Alpine was the top choice but I am not sure it would match your desire for a lightweight bivy. Numerous positives and I strongly considered it but realized my solo tent would fit in the same space and be much easier when the inevitable "call of nature" demands action. Be advised, REI wants like $250 for the Gore Tex wonder but Amazon has it for $140.
Well, so much for $140........this was only a few weeks back but now it's $46 more, still better than $249.
OR Alpine Bivy