Pretty warm for the low bulk
I spend most of my winters near Syracuse where snowfall can reach triple digits and temperatures plunge well below zero at times.
I'm 5'8" and 135 lbs and I wear a small and I can still fit a sweatshirt under it.
I previously had the Patagonia Down Sweater until it was stolen from me. I would consider the zonal nearly as warm and much less bulky. For this reason, I think the Zonal is superior. I haven't tested its waterproofness as I usually layer it under a Mountain Hardwear Drystein jacket, I highly recommend that as well.
I do wish It had a hood though for those days when I don't want to ruin my beautiful flowing locks with a beanie.