In reply to glenn0010:
Another vote for Salomon. Some say they're expensive but it's a quality, specialised bit of equipment that you don't want falling on you or chafing in what can be a hard enough challenge as it is. I used to use UD for years but swapped brands when it was time to replace my old AK3.0
I use the new Salomon ADV Skin 12 because in an ultra I'd rather have space to spare and you can cinch in the pack so stuff stays secure. The pole carrying system although a little confusing at first is very flexible.
The latest version is very stretchy and great for easily stuffing things in and out while on the move. I wore it for 42 hours straight during the UTMB and had no issues with comfort whatsoever.
I'm thinking of buying a smaller one for shorter races.