If you don't mind spending a lot of money for a top-notch bag, look at the Mont'Bell Spiral Down bags. The spiral cut is extremely comfortable. You can sleep in varying positions because the bag stretches with you. Their best 0 degree bag has 800 down fill, has the water-resistant DWR coating, weighs 3 lbs 3 oz for the long size and will set you back $530. - http://www.montbell.us/special/produ...l-down-hugger/
Outdoor Gear Labs rates the Feathered Friends Snow Bunting bag the highest 0 degree bag, saying it was warmer than the MB bag even with less down. In the long size it is 2 lbs 15 oz, has 850 fill, more features and runs $599.
If you want the Kelty Ignite Dridown bag, Cabela's has the long on sale today for $195.