Yes its light for its warmth
Also packs very small compared to fleece (perhaps the bigger difference actually.).
Hence being great for packable insulation you throw on/off when you stop/warm up after a stop etc. Or even spares in case you misjudge the weather a touch. Or even walking slowly round cold towns.
The garments are windproof because of the two layers of windproof fabric sandwiching the insulation. Having two layers is actually one of the problems - ideally you only want maximum air flow right up to your outer layer.
Thus open structured fleece with a windproof layer on the top ideal - whether you sew it all together a matter of taste etc. Also the downside of compressability tends to be durability after repeated compression (not great.). Outer fabrics also often too light for something you're planning to wear but thats not intrisic of course.
Shrug. Its hardly the worst idea people have ever tried to market! Still it does seem slightly silly. The durability over time bit does annoy me very slightly mind.
Of course if you're really trying to pack small for hand luggage for aircraft, or a really long trip through lots of climate zones etc then OK 
Posted: 30/01/2011 at 19:21