In reply to Mike Nolan:
Have a Minim 500 which they only offer as a special in sales. The first one I sent back because it had a cold area at thigh level. The replacement is much better. Also have a Minim 800 which I've used up to 7000m.
IMO they are great bags, but I guess the majority of people would hate not having a zip. I like the simplicity and the weight saving.
If you are of the larger persuasion you may find it a bit claustrophobic, the 800 especially is quite snug.