I’ve become a convert to what I’m calling (and with all due homage to HPG) a wilderness serape.  A synthetic blanket/overbag/poncho with a light, but not too light, DWR coated nylon shell.  You can find the specs on the ugly one I made this past autumn here.
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It’s been indispensable ever since.

The serape is one of those items whose function is beyond mere weight.  At 23 ounces, it’s 20 degrees colder than a good down bag of the same weight, and hardly lighter than a DAS parka, with almost twice the insulation in the torso.  Using one might make your pack lighter, but you’ll have to think about it.

As an overbag, a serape will not provide better warmth than a down bag of the equivalent weight.  Over a down bag which is slightly lighter than might otherwise be prudent, a serape will make a sleep system warm enough, and most importantly absorb some of the moisture which would otherwise end up as frost inside your down bag.  Under the right circumstances, on a colder trip with no opportunity to dry gear for days on end, such a system could be lightest.

As a coat, the serape leaves a lot to be desired.  Wearing one while doing anything but walking in easy terrain is impractical, as is doing so out under more than a decent drizzle.  You’ll need additional insulation for such purposes.  The virtue of the serape is that it traps lots of air underneath, effecting more warmth than the equivalent insulation in a close fitting jacket.  It is also handy to have this insulation serve two purposes.

For most of the winter I’ve been bringing the serape as part of my sleep system, and a fleece or synthetic fill jacket that’s a good bit lighter than I would have otherwise brought.  The jacket is handy for layering and wearing on the go when it’s really cold.  The serape is handy for throwing over moving layers for short breaks, and for layering with the jacket for longer ones.
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Today, I hauled it along for a break while mountain biking.  My toes got a bit cold before it was time for a beer break, so I pulled the foam mat out of my pack, took off my shoes, and wrapped up.  It’s just big enough to seal body heat in, and made for a very pleasant break.
The only alteration I made to the first version was adding full velcro to the neck opening.  The amount of insulation is a good balance, and the quality of the DWR fabric is excellent.  Going with something not so light (30D), is a good idea for something that will get sat on.  I’m sure it will remain a frequent item in my pack year round.