My first lightweight sleeping bag was the Helium EQ, which I still have and now use for winter/really cold temps. (I did take it with me onto my Mt. Whitney trip last year).
Note: the EQ is heavier with a full length zipper and waterproof shell. (Mine is 2007 or 2008 model at 2 lb 3 oz).
It is a great bag, love it and don't plan on ever replacing it.
The cut is wide, which can be one of the criticisms of the bag.
The inner fabric of the bag, I believe is a Pertex?, which is silky smooth and the hood closure does not use velcro.
The whole line of Marrmot 850+ fill down bags (light flight series?) won the Gold Award from backpacker mag a few years ago, since it kept score consistantly high marks.
Down side of the bag might be that they could have saved some weight with lighter shell fabrics.
For me, this is my winter bag or for any situation where I know that I am going to facing sub freezing temps on a consistant basis.
If I only had one bag from Marrmot, I think it would be the 30F Hydrogen, which was reviewed here on BPL as very warm and true to temp rating, and then layer up the clothing to get to the lower temps.
I have used mine down to 5 Degrees F in the snow and layering up, I was toasty warm.
Hope this helps.