Pretty, pretty, pretty good.
If you are looking for a well-constructed, warm, down jacket this is the stop to get off at. I use the MH Nitrous as a insulating layer for skiing. The jacket stuffs into my pack easily and is there to keep me cozy when the cold sets in.
I comparison shopped this with the ever popular Patagonia Down Sweater. It has a more athletic fit than the Patagonia which felt really "boxy." I also liked that the Nitrous jacket is two toned; the exterior red is a bit muted and contrasts the bright red interior. The Patagonia is all one tone and a bit drab. The one advantage the Patagonia has is that it has a pocket you can pack the jacket into. The Nitrous does not have a pocket and does not come with a sack to stuff it into.
I am giving the Mountain Hardwear Nitrous 4 outta 5 stars (lack of a packable pocket or sack knocks it down a notch from perfect.)