I carry a bivy, and there's nothing like it. being able to lay down in a rainstorm and not be wet or cold, even without other shelter kicks ass.
BUT, you absolutely get what you pay for. cheap plastic will gaurantee major breath condensation and a moist, if not downright wet morning. gore-tex, while a little on the heavy side is the only way to go in my opinion. You just can't beat the bivy bag from the military's Modular Sleep System (MSS)
GORE-TEX BIVY SLEEPING BAG COVER BIVEY NEW Military MSS - eBay (item 350328611424 end time Apr-13-10 19:14:52 PDT)
it is seam sealed, breathable, yet completely waterproof. Personally I choose to carry the whole MSS system, since the mummy bags snap together and guarantee you're just plain not gonna freeze. at 8lbs, it is a tad on the heavy side, but no worse than you'll get if you try packing around a small tent and a regular sleeping bag. and it's durable. You would have to want to destroy it. I'll take that weight, cause I'm hard on my gear. this system and my poncho (military OD) can't be beat for mobility, versatility, and durability.
Since it is 3/4 zip down the side, and has big-toothed zippers, I find it pretty easy to get out of in a hurry. Certainly no more difficult than any mummy bag. just grab the fabric on either side of the zipper and pull them apart. have done several 'emergency' wakeups (i.e.- asshole in my camp) and haven't busted the zipper yet!
My only complaints are: I wish it were a tad bigger, since I'm a big guy. And you can't take your grip in with you like a tent, but that doesn't matter all that much to me since my bag doubles as my pillow.
they are expensive, and you'll pay $300 for a used one at any surplus store, IF he's got any. But keep your eye peeled and you never know. I got my whole system, new in the bag, for $90 at a gun show! I know that sounds ridiculously expensive, but it's not- it'll be the last you buy.