After going through several down jackets in the last few years (TNF, MH, Marmot, Moonstone,) I was looking for a lightweight down jackt that offered maximum warmth in regards to weight.
When I heard of the limited edition Quantum Nuptse, with Pertex, I had to have it. Already owning a regular Nuptse, I thought the slimmer profile and lighter weight would make an ace in my outdoor arsenal.
While it is lighter, and the Pertex feels great against the skin (in addition to providing better weather protection,) I can't feel a huge difference in the insulation factor b/w this 900 fill jacket and other 650-800 fill jackets I have.
Also, make no mistake, this is a very puffy jacket. The pictures might make it look trim fitted, but it lofts up like no other, which is a good thing for warmth, however b/c of this it is really quite hard to layer with a shell.
Lastly, beware of the sizing. I am 6'1", 167 pounds, and wear a medium regular Nuptse, which barely gets across my wrist. It is pretty form fitting, but I like it like that (the large is too big in the waist and chest, and makes me look like a huge marshmallow.) The Quantum seems to run smaller in size than the regular Nuptse (at least an inch in sleeve, and 1-2 inches in overall length.) Make sure you take this into consideration. I found this to be true with 2 different M sized jackets...
Bottom Line: A good jacket which offers premium features including 900 fill and a Pertex shell. However, you have to ask yourself it is worth the extra $100.