This is a great jacket, period. It packs away like a dream, super light-weight and is basically waterproof. I chose the Lithium color in men's medium. I bought both medium and large and the large was way too big. The medium fits very athletically and doesn't leave a ton of room for layering ... A 100g fleece/T-shirt or less is about it. If you choose this coat strictly for belaying, I'd definitely size up.
As far as CoreLoft goes, it's warm, but I find PrimaLoft Gold warmer. It takes some time for the jacket to retain body heat. It's not like wearing a down piece where you feel your radiant body hear almost immediately.
As far as the hood goes, I feel Arc'teryx dropped the ball here. Usually, Arc'teryx is the king of hood construction; Not here. The hood has only 40g of CoreLoft compared to the rest of the jacket at 100g's. The hood has a very odd fit and I find it difficult to get a proper fit without a climbing helmet. There is a ton of excess outer (Windstopper)material and when wearing without a helmet, it makes you look very goofy.
The inside storage pocket is great, just wish there were two instead one just one. The back of the jacket does reach close to the bottom of your bottom, which is a nice touch. The cuffs fit very tightly, making wearing with certain types of gloves challenging. This does keep blowing wind out perfectly though.
Personally, I wish Arc'teryx would update the hood - fill it with 60 or 80g's of CoreLoft and adjust the cut so that it fits better when not wearing with a helmet. Four stars for fit and hood discrepancies. Great over-all winter jacket, just size up to fit over your action suit for belay purposes.