Not a review as such, but here's my thoughts on my Alpkit bag. I have an AD500. The first thing to note is that it's a down bag and has all the pros and cons associated with a down filling.
The major negative point quoted about down, is that it loses its insulative properties when wet. I don't doubt for a minute that this is true, but if your sleeping bag is wet, you're doing something seriously wrong, so it's a bit of a non-issue for me.
The AD500 is very warm. I got the 500 as I figured I'd get more use out of that than a higher rated one. If it's really cold I can add layers of clothing or if it's really, really cold use my arctic bag instead. The AD500 weighs very little and compacts down nice and small, so takes up little room in your rucsac. I haven't used it below freezing yet, but I did use it in early spring and think that the ratings are probably reasonably accurate. As I've already noted in other posts, I'm not a fan of Snugpak bags at all, and have no experience of Nanok bags.
In terms of customer service, I've found that Alpkit really takes some beating. Delivery is usually next day and the bag comes with a big cotton bag for storage as well as a compression sack for transport. I also have the hunka bivi bag, which I've only recently bought and haven't tried yet, but that's also very light and is the perfect accompaniment to ensure the sleeping bag never gets wet.
HTH.
