Without doubt, this is a "must have"
When a product already has so many positive reviews, I usually see little use in jumping on the pile. But this is such a killer product that I can't help myself.
Yes, it is ridiculously light, and packs very small. But it is the perfect combination of warmth with some breathability. Usually, I tend to run on the warm side, so I'm often putting on then taking off layers. But (to give you an example) yesterday, I put it on and left it on all day. In the morning, it was 25 degrees and breezy, and I was fine. Then, the wind picked up and snow started falling, and I was comfortable just by putting a lightweight shell on top of it. When I came inside, I removed the shell, yet didn't overheat. And I then wore it during a long drive and still remained quite comfortable.
Truthfully, with the Nano Puff, I could probably get rid of some of my mid/heavy weight fleece pullovers, and maybe even a softshell or two.