Rating: 5 / 5
This is a really special piece of gear. My size medium weighs 6.2 ounces and is approximately as warm as a fleece vest, but with better wet-weather performance and superior thermal regulation.
Possumdown, for those unfamiliar with it, is a blended weave of possum fur and merino wool which tends to have more loft for the weight than pure merino wool and more durability than pure possum fur. It is quite warm, soft, and compressible. Don't expect the same thickness as a good fleece, but do expect some very low weights and good warmth. I think the warmth is about comparable with Patagonia's R1 fleece, maybe a tad less warm.
Possumdown is not as bomber as fleece, however. For one, it's dry clean only, which might complicate things for a thru hiker who gets his possumdown muddy. The fabric also tends to become less soft and perhaps less lofty when you stuff it into the bottom of a backpack. I don't think this affects the warmth, though.
I have used this vest as part of a cold weather layering system, both in active use on cold mornings, sleeping-wear, and camp-wear. It really does work well for all of those uses because merino wool tends to regulate temperature quite well (i.e. when it's cold out, it feels warm, and when you're active, it vents off enough of your sweat and body heat to stay comfortable). What I'm most excited about is using it as my main insulating layer on late spring trips where a touch of insulation is nice to have at night, but a fleece vest would be too heavy or bulky.