The Patagonia Nano Puff pullover arrived via a friend on a trip to the US recently and will become my insulating layer and camp wear for some time to come. At 268g on my scales, the same weight as a 100 weight fleece or 200g/sqm merino base-layer, the outer is made of wind-resistant, moisture-shedding 'Deluge' DWR finished 100% all-recycled polyester shell (15 denier ripstop - roughly half the strength of Pertex Microlight) and the lining (also DWR treated) is 22 denier nylon. The insulation fill is low-bulk, hydrophobic, highly compressible 60g PrimaLoft One which Patagonia remark "...traps heat with remarkable efficiency, even when wet". It's a claim often made by Primaloft garments and it is certainly going to be more effective when wet than down, but it's important not to overestimate Primaloft's ability to retain warmth when wet. The deep front zipper allows for easy, effective ventilation.
The Nano Puff also packs down to a small 15cm x 10cm x 6cm package and stuffs into a single chest pocket that has a carabiner clip-in loop. Unlike the Montane Fireball Smock, there is no beard-guard which is a shame as that is pleasant feature. The wrists and waist are elasticated lycra and simple with no drawcord loops - I prefer that as adjustments are unnecessary are something this lightweight and therefore unnecessary weight.
It's neither snug nor loose, but somewhere in between, so can be used comfortably in a layering system with more than one layer beneath it, but is sufficiently close-fitting to be worn in a sleeping bag. It is not warm enough, in my view, for true 3-season use without something else like a second merino base/mid layer, but it sits alone with a single base-layer during late spring to early autumn. I may well use it in my WM Summerlite for sharp drops in temperature in the spring and summer months, and as part of a layering system in winter. It has the advantage of being capable of being used on the move as well as a quick rest stop pull-on. All in all, I like it and it will be the cause of me ditching a fleece for some time to come.