For three years, this was my absolute go to piece, then I got the Patagonia Nano Air hoody and this Nano Puff has fallen to my second most reached for layer.
While the Nano Air breathes quite well for a midlayer, this Nano Puff does not, which is sometimes a very good thing. I run hot, and if hiking or climbing in cold temps, I can work up a sweat wearing very little all the way down past freezing. So if on a climb, the Nano Air would be a much better choice for me at a belay station, where I'm subject to get chilly from the wind while I sit for maybe an hour. Similarly, while hiking, I may be in a tshirt or thin baselayer while moving, but when we stop to eat or regroup I want to trap all that heat I've worked for, and not let the wind steal it all. I still bring the Nano Puff for both of these situations.
The Nano Puff packs down incredibly small for a windproof, synthetic piece. That makes it a prime contender on summer backpacking trips to the alpine, where we know we may see freezing temps overnight or cold and windy storms during the days. While being coated with DWR, and windproof, this isn't a rain shell and will need to be layered properly in a downpour lest it wet out. Luckily, being synthetic, it'll still keep you warm when wet, but it definitely won't be comfortable and you'll have to wear it dry in your tent for hours. This quality makes it an awesome emergency piece, a just in case safety net that weighs 3/4 of a pound but will cut wind or cold in any scenario.
Mine has been incredibly durable, 5 years in now, with only a few small patches from wear and abuse. I take good care of it, but I don't baby it like I am known to do with some down jackets.
The fit on mine, again a model from many seasons back, is smaller than my Nano Air from 2015. The cut is a bit short, and the arms are too, but I still like the look more than sizing up and swimming in the chest. I'm unsure if Patagonia has changed the cut since, so can't speak to the newer versions.
This is a great, practical midlayer that fills many holes in the wardrobe. It is not a breathable midlayer, if you keep that in mind, you will not be disappointed, as it will keep you warm and the wind at bay.