I try and operate under the multi-use philosophy. That's to say I try and minimize the single use items I carry. A bivy, imho, is a generally a single use item and has little utility other than a mediocre place to put your bag and sleep.
If you're going for a bivy to support a tactical use then I'd suggest one of the older issue Ecotats. These make a solid bivy/tent, jacket, poncho and a good poncho hooch if conditions allow. It's heavy but so are all the other "tactical" bivys out there....here's one on
eBay
I do occasionally carry a bivy for my fat ass civilian hiking if I think I'll be using shelters. The one I'm using currently is an
Equinox. It weighs 6.5 oz. To be candid I don't carry this all that often either because if I'm anticipating using a shelter I'll always carry a tarp in case the shelter is full so the bivy is redundant as I can use the tarp to keep my bag clean.
Here's a cottage industry bivy from
Borah gear. He makes great gear and this bivy is about 13oz. He also makes a cuben bivy at about 4.5oz!
Also consider a BA copper spur 1...not as tacticool but weighs the same as a tactical bivy and is a real tent. Think getting dressed in the pouring rain....or cooking...or....
Edit: here's another from
Outdoor Research (OR). They also make great gear.