Love this vest. I don't usually want extra warmth, but I think this is my most versatile safety/comfort layer. Stuck standing still longer than you planned? Temperature drop 15 degrees on you suddenly? Need a pillow to sleep on? This vest packs down very small and is super light. Stick it in a small stuff sack and it can be so many things for little weight or space penalty.

Recently I purchased a Micro Puff jacket and Micro Puff vest. Am currently using it on the AT hike. Temp dropped to low 20 F with snow flying horizontal, 40+ knot winds the night the cold front hit. I did layer both since I tend to be colder then my husband. I was toasty warm! At bedtime I wore my vest to help double my sleeping bag heat. It was perfect. Excellent investment! The minimal weight is just a bonus.

This is probably the piece of gear that makes me the happiest to use. So
light, so warm, so versatile. Keep it in a pocket and throw it on over a
shell when you stop moving, or wear it under a wind layer to really keep
the heat in. Roll it into a ball and use as a pillow (wrap it in some
other fabric first, though). And it's synthetic, so you don't have to
worry too much about getting it wet. Go up a size so it's easy to fit
over a few other layers.
My only complaint is that the built-in stuff sack is too hard to stuff
it in, and doesn't make a useful pocket when it's unstuffed (stuff falls
out). Also, a zipper might be easier to adjust than the snaps.
Note: this review is for the pullover vest, which weighs 6 oz.