I bought this jacket for a late spring backpacking trip to Yellowstone later this year. It's incredibly lightweight and packs down about the size of a nalgene bottle. I've worn it only once so far, as a tester. Don't expect to use this jacket as a windbreaker, but it warms up fast with exercise. On a moderate day-hike in 50 degrees, I was sweating and taking it off. I think it'll be just the thing for Yellowstone in the spring. SIZE: After reading many reviews of the 2010 version of this jacket I thought maybe MontBell would have gotten the sizing right for 2011. Not so. If you look at the size chart, it would seem that you could buy your normal size. Don't be fooled. Go one size up. I bought a M, when I'm usually between a S-M, and the M was too tight across the chest (I am a 34B) and the upper arms. The L is slightly too large, but I can move in it and layer under, if needed. Also, FYI: the "gunmetal gray" is actually more black than gray. The "persimmon" is basically blaze orange.