Bivy sacs 101
There has been some interest in/questions about bivy's lately so here's my 2 cents...
A bivy's main purpose is super lightweight shelter. It is one step more 'luxurious' than throwing your sleeping bag on the ground and the only reasons you would use one (rather than sleeping under the stars) are to protect yourself from the elements (rain, snow, bugs) or to add warmth.
Bivy's with poles are nice for folks who are a bit claustrophobic, and for those extended bivy situations (extended rain and no tents to hang out in). They are convenient in that it adds a little extra room / breathing space. Some also have poles for the feet though I don't really see any need for this. These typically have a 'head section' and unzips in the torso section for entry.
Pliable hood wire:
These bivys give you a little bit of breathing space though it may take a bit of messing with to bend the wire correctly to maximize the space. These have a similar floor plan as the Pole bivy's with the 'head section' and torso entry.
No wire, no pole:
Bare minimum bivy. Top entry or torso entry.
Torso entry will have a 'head section' and will have 1 or 2 zippers down the side or a single zipper down the middle (butterfly style).
Top entry bivys have a limited zipper on the head section and you have to slide down the whole sac for entry.
All bivy's will be waterproof (except bug bivy's), but the actual material used will vary. Gore-tex, eVent, Todd-tex, etc. Some are more waterproof, some are more durable, and some will breathe better than others. I won't go into the different materials used because there are so many and the arguments are endless. I will say be very careful where you set your bivy for the night b/c wear and tear on the bottom will eventually kill the waterproof properties (important if you plan on camping on snow). This doesn't necessarily mean you should use a footprint b/c rain may collect between the sac and the footprint which could be worse than laying on the ground.
Most will have some sort of mesh screen option for maximize venting while keeping out the bugs.
Weight: 1 - 2.5 lbs
Typically, bivy's with poles will be heavier but this is not a direct relationship. There are some models with poles that are lighter weight than other models without poles.
Packed size: 4X8 to 5X18
Typically poles will add to the packed size. Many times bivying is something you prepare for but don't plan on. You probably want your plan-B to take up as little space in your pack as possible.
The ghetto bivy: (how to rig your own in a pinch)
Empty your pack and slide your feet/legs in to the bottom with the back of the pack (the side with straps) on the ground. Put your shell (winter / rain jacket) over your torso so it overlaps your pack or if you're not using a sleeping bag just wear it. If you're using this setup you're probably concerned about weather and elements so make sure your torso is insulated from the ground even if it's pine boughs. You should also make sure all the other contents of your pack are protected somehow depending on the duration and intentions of the rest of your planned trip.
Most brands you recognize are fine...but the discount brands you should avoid. You do not want to cheap out on a piece of gear that you are depending on saving your @$$ when climbing/mountaineering plans go wrong. The following represents my opinion only...
Good: Bibler, Black Diamond, Integral Designs (mine has protected me for 8 years and is still kicking), Sierra Designs
Bad: REI (I only base this on experience with other gear of this brand), unknown brands (post, google, etc. to determine what is an 'unknown' brand)