The Sierra Designs Cloud 15 is a lighter and less featured version of the Marmot Plasma 15. Both have 900-fill down, vertical baffles, and Insotec Flow Technology.
The main difference between the two bags is their zipper lengths: the Plasma has a full zipper and the Cloud cuts it off at 40 in. Also significant, the Plasma has a full neck baffle that helps seal out cold air. With these two weight saving features, the Cloud is able to shave off a full two ounces from the Plasma's weight. In a world where every ounce counts, this is significant.
Another plus, the Cloud's vertical baffles and well-designed hood are extremely comfortable. The bag moves with you slightly better than those constructed with horizontal baffles, and the hood doesn't pinch your face when fully cinched.
Although we appreciate that the Cloud chops some weight off the Marmot Plasma 15, the half-length zipper radically reduces its all-season functionality. The bag is too warm to be used on hot summer nights (your feet and legs will roast), and you can't unzip it fully and use it as a quilt. This makes the Cloud better suited to multi-day mobile trips in cold conditions than all-purpose, do everything use. For many people, this will be a significant drawback.
The Cloud is the most expensive three-season down bag we've tested. Paying $500 or close to it is not a good value. Even the Plasma 15, which retails for a pricey $469, is a better buy. If you're looking to spend some cash and get the best three-season bag available go for a custom 900-fill bag from Feathered Friends. Their Hummingbird weighs 2.5 ounces less and is $80 cheaper than the Cloud. For the most warmth for your dollar get the Western Mountaineering Antelope MF, a top quality five-degree bag for $459 (at 39 ounces it's also very warm for its weight). In sum, the Cloud 15 is an overpriced top-quality bag. Your money will go farther elsewhere.
Spring and fall backpacking.
The Sierra Designs Cloud 15's quarter length zipper reduces weight, but also makes the bag ill-suited to warm weather and makes it harder to dry out.