Hands down, Marmot's Helium +15 sleeping bag is the best bag I have ever used and it has quickly become one of my most prized and critical pieces of gear.
Notes on my intended use:
- 3-season backpacking
- Emergency gear (plus a bivy) on all of my winter day hikes
- Possible 4-season backpacking with a liner + a winter bivy (will probably test this out car camping a few times as overnight winter temps here typically call for a minimum of a 0-degree bag if not a -10 or -20)
- I do not typically use this bag for car camping, opting instead to use one of my cheaper bags to minimize the wear and tear of my Helium
PROS:
- Excellent weight to warmth to loft ratio
- Almost perfect fit (5'10" with broad shoulders) -- roomy enough to be comfortable, but snug enough to efficiently trap and retain heat
- The hood is perfect! Normally I find the hood on a mummy bag to be quite uncomfortable. On really cold nights I like to cinch everything down really tight, leaving only a small hole for my mouth and nose to poke out. In most bags the cinch cords bite into my head and face, and the hood has an inadequate amount of loft and cushioning. The Helium's hood has excellent loft and cushioning, keeps my noggin, and by extension my body, nice and toasty, and has the most comfortable casing/padding for the two cinch cords.
- Excellent torso draft tube!
- Packs down very, very small with minimal compression.
- Mostly snag-less zipper
CONS:
- If I had to pick one, it would be that the zipper could be shortened to shave a bit of weight and gain a slight improvement in temperature rating. That said, it is a minor gripe, and not significant enough for me to lower the rating at all.
Overall: Excellent, excellent, excellent bag! 5 out of 5 stars.

I've own a 30 year old North Face "Cat's Meow", owned, and returned to REI, a North Face "Hightail", and recently acquired a Marmot Helium, to supplant the "Hightail".This Helium, for me is perfect for the non-hostile environments I expect to encounter while backpacking. After purchasing this, I spent a week,this past June, at the 8-10,000 ft. level of Bishop's Pass, and the high country of Yosemite. While I don't know the night time temperatures, my partner was cold in a +40 bag that he brought, no doubt wearing thermals. I was comfortable, in every way, temperature wise, and bag fitment wise.The zipper on this bag did not snag...unlike the "Hightail", which did too often, in "at home" living room floor, testing. The zipper on the Helium, is a full length, and it is perfect!I can only say good things about the Marmot Helium, and the small stuff sack that comes with it, leaves no extra room, as the bag compresses itself perfectly, with in it. Another reason I bought this, and returned the "Hightail", besides the "Hightail's" 1/3 length zipper, is that the Helium has a DWR coating on the shell...the "Hightail", does not....and I hate the 1/3rd length zipper of the "Hightail"!!!One last thing...especially for the REI administrators who will read this. This Marmot Helium...DID NOT win the Editor's Choice Award from Backpacker magazine in the April 2008 issue!It was the Marmot Helium EQ that won the award.[...]

This is definitely the best 3 season sleeping bag on the market. Compared to the Mountain Hardwear Phantom 15 and the North Face Hightail, it simply offers better warmth, more space, and lighter weight.
Super bomber zipper does not snag on accident. The shell fabric is lightweight and durable.
One should consider buying the Helium "EQ" version which has a waterproof/breatheable shell fabric. It costs a bit more, but is worth it in the long run.
The only downside, IMHO, is that the Helium is only available in Left Hand Zip.