Not all sleeping bags are created equal. Many outdoor gear companies temperature rate their bags much lower than the actual temperature you would be comfortable using the bag in. Marmot is one of the best brands when it comes to rating their sleeping bags accurately. In general a Marmot 15 degree bag is going to be much more comfortable at 15 degrees than say a North Face sleeping bag rated to 15 degres.
With that said, the Helium provides a warm 15 degree bag at a very low weight (less than 2 pounds). This is primarily a consequence of extremely high loft down (850+) (900 being the highest down loft currently available on the market). To top it off, it's built as a somewhat wider/roomier bag, which means that you can layer up in the bag for extra warmth and still be comfortable.
I own a lot of sleeping bags, including full fledged winter sleeping bags. None compare to the Helium in terms of loft or compressibility. Can't recommend highly enough.
I have had this bag for at least 4 years of light use. It has been worth every penny. Super light, and the lining feels amazing. I know it says 3 season / 15 degree, but when I pulled into Guadalupe State Park behind schedule we opted to just sleep in the bed of the truck. I woke up the next morning with a thin crust of ice on the outside of the bag, but was nice and toasty inside.
The reason I purchased this bag: After using a North Face Cat's Meow bag purchased in 1997 for many years, I went up Mt Magazine in Arkansas. I don't know what the temp was, but all night long stinging cold air was drafting in along the zipper. I had to try and sleep on the zipper to keep that out. So I went in search of a light weight bag with a full length zipper baffle, and have been in love ever since.
Make sure you store it in the big bag, not compressed in the stuff sack. This is what has kept it going strong for many years.
I don't know how they temperature-rate sleeping bags, but I know when I slept in this in the mid-20s I was barely warm enough. Maybe if I had been in a tent, or worn long underwear it might have been enough.
I am amazed at how light and how small it packs down. And it's nice that they include a large storage bag so I can keep it clean and loose between trips.
Update January, 2014: I recently hiked the John Muir Trail (220 miles through the high Sierra). I was so glad to have this as part of my ultra-light equipment, and I stayed warm at 12,000 feet. However, my brother had the same bag with the water resistant shell and I was jealous. Condensation soaked into my bag and I'm sure it reduced the insulation factor, while on his bag the water beaded up and ran off. If I had it to do again I would pay the extra $100 or so and get the water resistant model.