I'm reading excellent reviews of certain emergency bivvys that are a little fancier than just mylar and weigh between 4-6 oz, especially this one I found on Amazon: SE-EB122OR Emergency Sleeping Drawstring Bivy.

I was wondering if this could become my default bivvy, assuming that I would use it with my really good Western Mountaineering Alpinelite sleeping bag and my Therm-a-Rest NeoAir mattress.

Has anyone experience with this?

Thanks

The bivy you indicated does not look durable enough to use as anything but an emergency piece of survival gear. Odds are it would quickly disintegrate under repeated use.

It would be helpful if you were to link to those reviews you are reading, which would cite multiple choices and gives the reasons why you might consider them for non-emergency use, rather than just linking to the one.

Getting in and out of a bivy sack in the rain is not something you'll enjoy, especially when the rain comes inside as you're coming out, which it will! The other problem with a waterproof bivy is condensation of your body moisture (insensible perspiration) on the inside of the bivy, where it will wet out your sleeping bag insulation.

Better to use a breathable bivy in conjunction with a tarp.

Plus, as Aimless says, emergency bivies are meant for emergencies. They are not designed to hold up for more than a couple of nights.

I bought an early Gore-Tex bivy that was cordura on the bottom. One night on a winter trip in the White Mt.s of NH, I managed to roll around and put the nylon on the top. The bag got wet as the bivy trapped moisture. I think the bag was Hollowfill, so it didn't matter too much, and I dried it carefully with a Gaz backpacking tent heater. I think in certain conditions your down bag would get pretty damp and be hard to use a second night if you didn't have the time or conditions to dry it out. I'd also be highly skeptical of durability. Those things are meant for limited use. SOL makes a claimed breathable e-bivy, called a "Respirant" bivy, but you'd have to read the reviews. They produced it because of how notorious radiant bivy's are for trapping moisture.
I didn't mean to parrot what Oregon Mouse said, she posted hers while I was still composing mine. She hit the nail squarely, as did Aimless.

I have had good experience with a similar bivy bag, made by SOL. Their escape bivy is nice, and breathable, too. They make a mummy version and a lite version (no hood). I've used the mummy one quite a lot and it's help up extremely well. I just got the lite version. I haven't tried it out yet, but it's the exact same item minus the hood and side zipper.
There's also a non-breathable version if you're looking for a vapor barrier.

Hope this helps. Cheers!

Do you think they will last long enough to warrant the 60.oo price? I know you can find some breathable bivys for about 15 bucks more.

I got mine (the breathable versions) for around $35 dollars on Amazon. They are surprisingly durable and very warm for the weight. I only use one at a time. I only got a second one because it's lighter weight without the hood, drawstring and zipper.
I've used the non-breathable variety before. Never again. I sweat like a dog and wake up feeling miserable.

Wow!
Very reactive forum.
I'll probably give a try to the SOL Escape Lite, we'll see.
Thanks!