I was going to hold off on this since I have only used a couple of different bivys but the replies aren't exactly rolling in. I have an OR advanced bivy. Here's my thoughts on it:
Price: really high, probably too high
Ventilation: excellent, there are two poles which provide a couple of different options and keep the netting out of your face. the poles do not have to be used if you want to cut down on weight or if you are using it as an overbag.
Water resistance: Vinyl (I think) bottom, anf gore tex top, great.
Other factors: There isn't (or wasn't when I got mine) a zipper runnning down the side so you have to get in and out throught the top. This can be inconvenient until you get used to it.
It is very compact and light, but I suppose every bivy is.

Some additional thoughts on bivys:
They don't provide as much room as a lightweight tent will, ventilation will not be as good (generally), and they will be relatively expensive when compared to one.
I would recommend a bivy which has a way to keep the netting (which should be removable) out of your face when you sleep.
They can be used in cold weather to give you some additional warmth if your bag alone isn't sufficient. Also, if you have a down bag and are paranoid about getting it wet, the bivy will give you some peace of mind.
A friend has an REI bivy that has zips so you can get your arms out, this is a feature I was jealous of but you learn to make due.
A lightweight 2 man tent will have enough room for you and your gear if the weather gets bad, with a bivy you need to have someother way to keep it dry.
All this said, I am happy with the bivy I have but then again, I got it as a gift and didn't have to shell out the $230 (they're even more now) to buy it.
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