The Navis is one of our lowest scoring sleeping bags. At 21 ounces it's moderately lightweight and kept our testers warm into the forties. The bag suffers from numerous drawbacks that limit its performance and versatility: (1) it has a fixed girth that cannot accommodate changes in body type or clothing. Unless you're really skinny, a midweight down jacket won't be able to loft properly inside of it. (2) The lack of a zipper limits ventilation. Our testers turned the bag over and slept with the uninsulated side up during warmer nights. We much prefer bags with full length zippers or quilts because they can be used as a blanket on hot nights and can be draped around your body in camp. This bag can't do either of those. (3) The elasticized pad attachment straps are permanently fixed to the bag and are significantly heavier than attachment methods on other bags we've tested. They are successful in that they're very secure — even those who thrash about at night will remain on the pad — but unnecessary for most people. Several small webbing loops allow you to secure the pad with a thin cord; this is more versatile and lighter. If we used this sleeping bag more we'd cut the elasticized loops off.
Numerous other sleeping bags offer increased warmth to weight ratios, more comfort, and more versatility. See our full Backpacking Sleeping Bag Review.
The Therm-a-Rest Navis bottom has no insulation and its pad attachment straps add weight. Traditional quilts are more versatile, more comfortable, and warmer for their weight.