Adds extra warmth w/o weight or space
It's toasty, sometimes too toasty which is a good thing because it bundles down to a miniscule ball. I paired this with a mid-weight fleece, a base layer, and a hard shell for a summit at Kilimanjaro and it was perfect. Even had to take it off once the sun came up and we headed down. Great to stash in your pack as an easy way to add warmth.
As others have noted, is a little bit too bulky in the mid section. Other brands have got that nailed down better. This is often on sale, so don't settle for retail.