Ive always hated bivy bags as anything but an emergency shelter, or for use on a climb when weight and space for a tent is at an absolute premium. I hate them even more now that tents are so light, weighing maybe a pound more than a bivy bag.
Compare, if you will, Outdoor Researchs really very fine and good Alpine Bivy ($219), with MSRs one-person Hubba ($250). The Alpine Bivy weighs two pounds. The Hubba weighs...three. And you can sit up in the Hubba, something you really cant do in the Alpine Bivy even with its hoop-over-the-head area. Theres no contest, in my view.
True, you can find lighter bivy bags. Integral Designss Micro Bivy ($210), made with eVent, comes in at 18 ounces, which is pretty good. But I still wouldnt trade it for the comfort of a lightweight tent, except in the gravest of conditions. And then of course you can compare it to Mountain Hardwears Sprite 1 ($170), which can be trimmed to two pounds six ounces. Tents win.