I have both packs. My Arc Haul arrived this week, and goes on its maiden voyage over the holiday. I have used the Circuit for more than five years, and there are three of them in the house....
Some thoughts after initial loading of the new pack:
1. They are both well made, as I would expect.
2. The Arc Haul appears to have a larger main compartment. It's made entirely of the Dyneema fabric, and is totally seam taped on the inside (!!!). The Arc Haul also has a better side compression system (similar to the ULA Ohm). The larger main bag cuts both ways, of course -- I won't be able to fill it during summer hikes, but it will work great in the winter, and I am trying to own only one pack.
3. I think I am going to love the Arc system, which gets the pack body away from my back. The Circuit is a very sweaty pack on me, my back is always soaking wet even in the dead of winter.
4. I got the "Tall" Arc Haul, having used Tall ULA packs. According to Zpacks, there is no difference in the pack body itself, but the frame is a couple of inches longer. I'm not that tall (only 6-1) but I have a long torso. The Arc Haul fits much better than the Circuit, as it puts the top of the frame just above my shoulders. The load lifters work much better in that position. If you are trying to figure out what size to order, email Zpacks, they are very helpful.
If I were to totally fill the pack all the way to the top, it would look like it does in the photos on the Zpacks page, coming up to the middle of Joe's head. I don't expect I will fill it that far
5. All the webbing on the Arc Haul is very narrow and somewhat stiff, and doesn't move through the buckles very easily. So, for example, trying to loosen the load lifters isn't all that easy. I'm assuming this will get better with use. Also, the webbing that makes up the lower half of the shoulder strap is too short on my pack, though it is easily user-replaceable. (In fact, the Zpacks stuff is designed so that pretty much any part can be removed or replaced withiut cutting it off -- I like that.)
6. Comfort -- the Arc Haul feels great in my living room, but beyond that I can't comment. I will say I like the full wrap hip belt, and the overall suspension feels good. Given a very cursory test drive, it does feel like it will handle the ~30 lb loads leaving town somewhat better than my Circuit, though that was no slouch.
I've been beating on my current Circuit for four or five years now, and it's time to replace it. (For one thing, much of the fabric was delaminating and I couldn't keep water out on our LT hike last summer.) I was planning to get a new Circuit, a custom job with all Robic fabric and the little features that I like (e.g. the stretchy front pocket), but the Arc Haul just called my name for some reason. We'll see how it goes next week and provide an updated report.