Well, I've been doing some research all night about this but I still feel ambiguous about buying a bivy.

They seem durable, light-weight, comfortable, waterproof, and all that. However, how well do they really get the job done at higher altitudes/snowstorms/climbing trips/etc?

My main concern is how do you get in without getting the inside soaked during a snowstorm or rain? How do you fit your pack and what do you do with your wet clothes and boots?
Would it be reasonable to have a bivy as your main shelter for climbing trips?

Also, what kind of sleeping bag should I get along with a bivy? (I'm looking at the Outdoor Research Alpine Bivy). Is a +10 bag suitable for most weather you get in the Sierra Nevada's for example? Or would a 0 or -10 be better?

I appreciate the help guys. I'm just a really picky shopper..

I have an Integral Designs Endurance Bivy. It is really nothing more than a sack, no frame, insulation, or structure of any kind and weighs well under one pound. However, it is made of event material and is waterproof and windproof. In cool, clear weather, I sometimes use it as a sleeping bag cover when I want to sleep out under the stars. In harsher conditions and on expeditions, I always carry it in my pack as an emergency backup shelter.

However, I prefer a tent rather than a bivy sack for most mountaineering and hiking trips. A light weight, sturdy, one or two person tent gives you a lot more options than a bivy and is far more comfortable. Even a small tent will allow you to store your pack inside, keep your clothes dry and make eating and dressing easy compared to a bivy in any kind of bad weather. I have a two person, four season mountaineering tent that weighs about 5 pounds including poles, etc. You can get tents that are much lighter depending on how you intend to use them.

I have a bivi sack, it is a Bibler, well made, reasonably light, and made with Bibler's proprietary fabric Todd-Tex which is basically the same as Gore-Tex. I find it does not pass moisture well enough and I always end up with a wet bag. I can fit my boots in but not my pack, stove, or anything else.

There are so many excellent light weight tents that offer much more protection and comfort for not much more weight. The only time I would ever use a bivi sack is on a big, technical alpine route where it would be difficult to find/or excavate tent platforms and then I would bring a synthetic sleeping bag to deal with the moisture issue.

If you are thinking about hiking/backpacking, I have been super happy with floorless shelters, aka tarps. I use a Black Diamond Betamid. For $99.00 and about 2 1/2 pounds I can fit 2 people and our gear. It is surprisingly storm worthy and I have ridden out some doozies in it. I have used it in high camp on Rainier multiple times, Baker, Shasta, as well as countless other peaks in the Cascades. If you want to save on weight they make a Sil-Nylon version that weighs about half as much. Many other companies make similar products.

In short, IMHO bivi sacks are specialty items that are superior in only very specific circumstances i.e. technical alpine routes, emergency 'just in case' shelter, etc.

Alright then, I'll just opt for a small tent instead.

I appreciate your feedback guys! Thanks a lot!