I just got home today from a 11-day trip in the Sierra with a bivy. The first two days it drizzled from 2PM until dark. The rest of the trip was nothing but clear blue skies, one night with frost, the rest fairly warm. Most days were very windy because I camped between 10,800 and 12,000 feet every night.

I have the MSR basic bivy. The key for me is that it has little loops that with three tent stakes I can basically set the base of the bivy. This is important because I like to sleep on an incline and this way I do not slide down the hill! It also has a loop on the top (head area) so that I can tie it up on my trekking poles. Thus, I have a very little tent. The bivy stays in place when I roll around at night. I also take a small tarp that is either a ground cover when I do not expect rain, or a "vestibule" when I expect rain so that I can cook while in the bivy. As far as rainproof, I have been totally dry in major downpours with just the bivy.

Conclusion - the bivy worked well. Because I set it up using my trekking poles and I camp at windy locations (to avoid bugs) I do not have condensation problems. I enjoyed looking at stars at night. With 11 days food and a bear cannister, the bivy is nice and compact and fits in my mid-sized pack. The tarp set up for a drizzle is a bit fussy. I had to find just the right rock with a flat sandy spot next to it to get it all set up right. If I had to depend on a tarp only, I would find it very tedious when I camp primarily above timber and often at places where there is not even a big rock to attach a tarp to. And I ofen camp in very windy spots.

If you are going to spend considerable time in camp, I would not use a bivy. If you are going to hike most of the day and just crash at night, a bivy works well in a climate like the Sierra. The advantage of a bivy over just a tarp, is that you do not get wet from underneath or horizontal wind driven rain. That said, if I expect it to rain every day, I would use a tent.

A bivy is not for everyone. You really have to LIKE being out there in the elements.