I was looking for one last year and ended up with the Montbell UL down jacket, mainly because Mammoth Mountaineering happened to have killer deals on them at the time and I was in the store browsing. I probably would have bought the EX, but they where sold out. After a year, and one thru-hike of the JMT, I'm glad I bought the one that I did. Its a little more durable, has DWR, and pockets. Mine weighs 8.1 oz in the large size which is half the weight of my Columbia fleece jacket and packs to the size of a grapefruit without over packing it. Its warm, I've had it on under my quilt on some of the colder nights, and I keep the stuff sack in the pocket when I use it. I still go back and forth as to whether or not I should have gotten the parka for the hood, but in reality I would be carrying a fleece lined beanie anyway for when I want my head covered but don't need the jacket (such as when hiking), so I decided against it at the time.
I've used it under my pack quite a few times, including on the JMT, and will continue in the future. In reality though, that's not a whole lot of total time as I rarely end up needing a jacket while actually hiking. Its usually just putting it on when starting out in the morning till I warm up, then it goes in the pack. But its comfortable and fairly breathable when I do. It's kept me warm into the mid 30's in camp, I'm guessing well down into the 20's while actually hiking although I haven't had it out in weather that cold. I'm really not worried about it breaking down the fill under the jacket just by wearing it under a pack. It may eventually do so, but I've used other down jackets under packs a lot more than this one with no noticeable loss in loft or comfort. But overall, I consider this to be one of the best backpacking investments I've made.