I've been loving my Nano Puff. Talk about the perfect piece of insulation! I should also mention that this piece does have a Durable Water Repellent finish (DWR), so light rain and moisture will not penetrate it.
In a heavy down pour it definitely will soak through. Ounce for ounce you'll find the Nano to be warmer than just about anything else out there. I find that because it is synthetic it doesn't tend to breath as well as the down products, but that factor does help with extra warmth.
Check it out and let me know what you think?