Selection: I selected the Spark III for a backpacking trip to Scotland because it gets a Comfort Limit rating of -4C, and although I didn't expect those temperatures in May I did expect to be camping quite high and temperatures could drop to 0C quite easily. Also I'm sleeping 'colder' these days as I get older and so I like to make sure I'm going to be warm without resorting to wearing lots of clothing.
With 400grams of 850+ Fill Power down the rating seems about right, its been tested using the latest standard and compares with other bags with similar amounts of down in them.
If you compare this with other bags that use 400grams of down you can see why the Spark is special - it weighs just 642grams in total, whereas other 400 Fill bags weigh up to 200 grams more. So I could get the insulation value that I wanted at the weight of a much lighter bag.
Performance: I like this bag very much. In order to make it so light there are a few compromises but size doesn't seem to be one of them, of course its slim, but it didn't feel unduly cramped except slightly round the shoulders.
A few of the nights were cold but I was warm as toast throughout, and slept really well. The Spark only has a half length zip which is fine for getting in and out of but you can't vent the bag from the bottom if it gets too warm. Its also a one way zip, so you can only zip it down from the shoulder.
There's no shoulder baffle and I found that without the hood fastened up, if you moved, the warm air from inside the bag rushed out the top! Cinching in the hood section eliminates this though. The zip has a baffle to eliminate cold spots and the foot is boxed for comfort in this area.
When using this bag I slept on a Thermarest Xtherm, a very warm sleeping mat which also boosted the rating of my sleep system.
Other Comments: In addition to low weight the bag comes with a minute compression sack which is exactly the right size to allow maximum volume reduction for packing.
The Spark is a fabulous series of sleeping bags and whilst there is some compromise on features to keep the weight down I would not advocate changing a single thing - keep it as it is - a great minimalist bag that achieves exceptional warmth given its very low overall weight.