I used the ULA Circuit for my John Muir Trail hike and loved the pack. It had more than the volume I needed and carried the weight very well. I had a ~16 pound base weight with total weight ranging from 23 to 33 pounds (with the 33 pounds after my final resupply at Muir Trail Ranch). Even at over 30 pounds, the pack was very solid.
As much as I like the Circuit, however, it does weigh in at 38 ounces and I'm looking to get my base weight down from 16 pounds to around 12 pounds before next summer. I have a zPacks Hexamid and love the shelter and would like to buy more zPacks gear now that I'm confident in the cuben fiber material. The ArcBlast, with the configuration I would want, should weigh in at under 20 ounces meaning I could save well over a pound if I switch. I am considering switching to a zPacks sleeping bag which will save me another 18 ounces or so. Since I probably won't need a bear canister for my hiking next year, that will save a couple of pounds getting my base weight down to 12 pounds. I would then expect my total weight to be between 20-28 pounds almost all the time, below the 30 pound limit advertised for the Arc Blast.
If anyone has owned both the Circuit and the Arc Blast, I would really appreciate feedback on how the two compare. It seems intuitive to me that cutting almost half the weight on a backpack has to involve some compromises in terms of comfort ... however, I have found in other areas that cutting weight does not necessarily result in less comfort so I am wary of relying on "conventional" wisdom when making my choices. Although I don't have an unlimited gear budget, I am prepared to spend some money to upgrade my gear with a focus on going fast and light next year.