I have used the Arrakis 40 and 65 extensively. Other Arcteryx packs used is a Bora, I was somewhat hesitant at first at changing to the unconventional pack design, but I am so glad I did.
For a full idea of pack size, the 40 is perfect for day trips, and single day snowboard or cross country ski trips, while the 65 is more of an expedition pack, that I have thus far crammed 1 week of supplies and sustinance into.
This pack does not have a side zipper like the larger models, this makes it a TRUE drybag, so for full submersion activites, ie, deep water soloing, kayaking/rafting..etc this is the backpack of choice. The side zip on larger models will not leak during downpours, (tested in the monsoons of southeast asia), it will slow drip in full submersion.
Material is tough, and I mean, REALLY tough. I take very good care of my gear but have never had any inhibitions or worries when tossing this bag around. My Arrakis 65 has been tossed of small cliffs, dragged in the water, left in the snow overnight, for 2 years now, and still looks brand new. Also the material is very easy to clean.
Pack Design, originally my main worry, now I can't imagine having ever used a different pack, while most hiking packs have a stacked design with the pack space becoming narrow at the bottom, the arrakis is slightly wider at the bottom and allows the pack to be organized in such a manner that you can pull items from the bottom without unpacking the bag. This is very convenient.
The pack does not have compression straps, nor could it really, the material wouldn't compress much. The pack design inhibits the flopping around a normal pack would have if underpacked and not compressed. And the "3D" molding design of the pack makes the pack pretty much sit and look the same regardless of what's in it.
The rotating hip waistband is everything arcteryx claims it to be, wonderful. It moves with your body like a latin dancer. From a past of everything in packs from jansport to northface to marmot, it's the comfort that keeps me wearing arcteryx on long hikes.
Straps are sufficient, I only say this because I personally added some more and sometimes tend to strap too much to the outside of my pack. But for the normal person, the 40 accomodates a snowboard or ski's wonderfully along with a mountaineering axe or other small accessories. Or if you climb it's easy to strap a rope to the outside.
Hop on the Arcteryx site for the detailed size charts, everyone I know (including myself) that has followed the chart has been happy with the outcome.
Great pack, you'll love owning one, in any size.