In reply to jacobb:
I have a Hunka XL. Its works well as a bag cover. If you bivi in summer you'll get eaten by midges, so need a face covering.
I also have the Alpkit hooped bivi. Its a bit bigger, and you could get through a rain storm in it, as it covers your head. You can use that without out the poles, and it has a midge net. As has been mentioned, if you need to do something, even change your socks, then a bivi is a faff. But if you just need to stop wherever, for a few hours kip, then a bivi bag is great.