I've had a 52 for a few months now, and have used it 5 or 6 times. Comments:
I was oppposed to having the long side zipper, just from the structural integrity standpoint. But I actually used it once to get something out of the very bottom of the pack in a snowstorm, and that was pretty nice.
It would be kind of nice to have an elasticized side pocket on BOTH sides.
The weight is good. The carrying capacity is good.
Fit is a sensitive issue with the Khamsin series. Not a lot of adjustability, so it either fits well in a particular size, or doesn't fit well at all.
The hip belt has been more than adequate for my needs (often around a 30lb load). It's also not so beefy that it gets in the way of a harness, and it doesn't get in the way when high-steppin'.
The load lifter strap buckles run through the hypalon stuff that connects the top lid, and they tend to get tangled up a bit.
With a lighter load, the pack can be compressed down nicely.
The "take the top off and use it as a small fanny pack" thing is actually more useful than other such ones that I've seen. On my Dana Terraplane, you have to take the whole freakin' main hip belt out of the pack to use it with the fanny pack/top lid.
Be watching for a sale; obviously, these aren't super-cheap packs. But you get what you pay for, as the Arc'Teryx construction is first-rate. I also considered the Osprey Ceres line of packs, but thought their "features" weren't really what I'd want.
The daisy chain attachment points on the top lid are kind of worthless unless that lid is stuffed full and has some shape. It would be kind of nice to get them on the body of the pack somewhere. I've run some shock cord through mine.
Adjustable ice axe loops and ski loops are nice.