I've actually used a mylar blanket one night at the high base camp in Mt Whitney, about 10K feet up. It held up find, and kept me warm enough at about 40 degrees (and dry). My Boy Scout troop was doing a fast ascent, and stopped for about 3 hours for a quick nap at the base camp. Once out of the wind (lots of stone walls set up over the years), the blankets worked great. Or course, noisy as heck, kind of hard to unwrap (they're REALLY well folded) and re-wrap. I didn't have any problems with mine tearing that night.

Now, this past winter, I tried to make a snow shelter with one in my backyard. It DID tear, but duct tape patched the hole, and yes, you do have to reinforce where you attach them.

FWIW, at that price, I say go for it. Even if they crap out on you, you have 10 nights worth of bags.