I love my Nano Puff PO. I pretty use it in in the same ways Eric talks about. It's the jacket I use the most- I take it on all car camping trips and most backpacking trips, it's the jacket I'll stuff into my jacket just in case on cooler spring/fall days, it's my casual jacket most winter days here in northern Minnesota (avg 10 F-30 F), and it's my warm up/rest jacket for winter hiking/runs.
For the kind of hiking I do, I rarely wear anything other than a windshirt and baselayer. Under 25 F I'll wear a FA Bat Hang Hoodie; above that, Ibex Indie Hoodie. While I'm warming up, I'll have the Nano Puff over my baselayer and windshirt. Once I've warmed up, it comes off and stays in my pack until I'm I'm resting. It has to be really cold (under 5 F, maybe under 10-15 F with very high winds) before I'll actually hike in insulation- and even then, it's in an Atom LT Hoody, which still isn't as warm as the Nano Puff.
I especially like having the PO vs the full zip- the slippery nylon and shorter zipper is perfect for slipping it on and off with the least amount of effort.
To each their own!