Compressibility
The Spark II’s wispy down is easily compressed using the included compression stuff sack into an extremely small package – just slightly larger than a 1-liter Nalgene bottle. 

Warmth
Although the Spark II is rated to 35 degrees F, I was uncomfortably cold in temperatures in the high 30s to low 40s; it was necessary to put on a heavy down jacket during those cold evenings. Better performance was found in the high 40s, in part owing to the fact that the bag is snug and does not have a lot of loft space.  

Overall Comfort
The Spark II provides a snug fit, without a lot of extra room or extras since its main purpose is to provide as efficient a bag as possible without adding weight. I do toss and turn quite a lot through the night and as a result I found the bag a little too snug, particularly when I had to put on extra clothing through colder nights. Nonetheless, I could still appreciate its billowy down. 

Durability
The Spark II showed no signs of wear and tear during the testing phase. The short zipper did snag a few times and, although I was careful to un-snag it, it might eventually result in some tearing or fraying at some point with continued use. The Spark II uses a 10-denier nylon fabric with a water repellent treatment to shed moisture on the outer bag, as well as a 15-denier nylon fabric to provide comfort next to the skin. These materials made for a very comfortable and cushiony bag, as well as one that is extremely attractive; the pale gray, semi-translucent shell reveals some of the down beneath, creating a very unique and elegant look.  

Features
The Spark II uses different baffle constructions over the legs and torso areas to lay out the down spaciously and efficiently, which resulted in the down staying where it was needed and made for a very comfortable and cozy bag all over the body. The Ultra-Dry Down® used in the bag is treated to be water repellent to improve down’s reaction to water without affecting weight or loft performance. 

The Spark II also uses a 1/3 length #3 YKK zipper that, although reducing weight, makes getting in and out of the bag more challenging, particularly in the middle of the night. I also have to wonder whether a 1/3 zipper is an appropriate choice for a 2-season bag that is more effective in mild climate – when one would more often need to unzip the bag further than the 1/3 length zipper allows when too warm. The strategy seems flawed, even if weight is the optimal goal. 

The Spark II has a wide fitting and comfortable hood that can be made tighter with the drawstring. Perhaps because there was no neck baffle, the hood felt snug and warm without making me feel claustrophobic, and I appreciated that lighter-style around the head. 

The Spark II is the most expensive in this category by a considerable margin.