The Thunder is a warm and functional down vest. It packs a lot of punch in it's weight to performance ratio- it is a thin vest but at 800 fill down, it keeps the core extremely warm. As per TNF, the fill type maximizes insulation where grams and warmth are crucial to success e.g. expeditions, long days outside. This vest works in any range of activities from running to mountaineering to backcountry skiing. If you buy the right size, it all fits correctly- I am 5'8 with an athletic build and a medium is perfect- in usage as both an outer layer and an underlayer. The cut and baffeling are very feminine, and very efficient. This vest wears well in high performance activities, and also transitions nicely into street wear. Design elements include 2 well placed pockets and a nice high collar to block wind and cold.
It features a water repellant finish which works great, and has "down show thru"- which means that because of the technical fabric, durability and compressability, the down can show through.

I bought this for my pregnant wife, seeing that the Large Nuptse vest was too small and bulky for her, so I got her the XL size... Still not zip-able due to baby but fits more comfortable for her. She tells me that her core temperature is definitely warmer after wearing it. The vest was a lot thinner than I expected. I figured with 800-fill, it would be a little thicker. The vest is very light in weight. We haven't tried yet, but I would assume that it will zip into a shell. She has received numerous compliments on it. Definitely a nice addition to the closet.

I am 5'7" and 125lbs and slim build. The XS in this vest was too short in the front for me. The arm holes were also cut fairly small and tight. This makes putting a hoody underneath very uncomfortable. I can see this vest working as a good layer underneath a jacket, but on it's own and layered overtop of sweater it's not great. The quality of the item, though, is very nice. The seams on the black color are taped in red and makes it look very nice on the inside.