This bag is awesome. It will take almost any bullet you can shoot at it--by bullet I mean serious abuse from sandstone, granite, etc.
If your looking for an ultralite bag this is not it, but then the muiras are designed for situations and abuse that would instantly vaporize an ultralite pack.
The features are fantastic. I love that the pack completely opens down the side so the whole front of the pack folds down. The gear slings on the inside are a great touch. The belt provides good support and holds the bag tight against your back without getting in the way. The roll top is just cool and is easier than unclipping and removing a top pouch to get into the main compartment. Also the roll top does not get in the way of the top pouch that is there. The removable rope straps are great for attaching lots of rope and it's nice that they can be moved from the back to the top of the pack for comfort or preference.
It is a fantastic bag for the approach hike and depending on how much climbing (or canyoning) gear and rope you need to take I find that 30-35 liters is good for between 2-4 days.
All in all i think it is my favorite bag of this size that I have ever come across.
Arc'teryx does it again . . . of course the Arc'teryx awesomeness also comes with an Arc'teryx price tag . . .