Ive owned 3 circuits. Still have 2.
What kind of load are you wanting to haul?
The niche it fills is a ~ 2-2.5 lb pack (strip the doo-dads off and med/small belt weighs 34oz)that comfortably carries 25-35 lbs. Largely due to the belt. It can carry up to 40 or so if needed for short durations. I dont think there is another pack in this range that is as comfortable, durable, sturdy, easy to use, and has the carry ability. It is as close to a perfect pack for long thru hikes as one pack can be. Thats not unusual, it was designed by a thru hiker, for exactly that.
IMO it is best in the sub 30 lb range, as are all light weight packs. If I carried 30+ most of the time, I would probably opt for a beefier pack. Bending the al stay to match your back best is critical to getting the most out of the pack.
The side pockets are really water bottle pockets. They are one of the reasons I worship these packs. Thats their design use. However tents, poles, and large objects can be carried in them fine. I often stick my hiking poles in them behind my water bottles and secure them with the velcro straps . There is a big gap at the front bottom where the shoulder strap goes thru the pocket, so ixnay on the little items, they will fall out. I can reach behind me, and remove a water bottle, and replace it while hiking. Quite a few people are not flexible enough to do that.
Many packs with pockets rimmed with elastic dont allow easy re-insertion of water bottles. I can do it on my Ohm too. When I hike with someone else with different packs, Im often having to remove their water bottles and replace them for them. I really get tired of that, they should just get a pack thats properly designed, or get a bladder. However I have had to do that for other hikers with circuits too, they just werent flexible or something.
Much of the time my Circuit is too big except very cold conditions. Id say its best in the 8-12 lb base range. I use my Ohm most of the time when I have a 5.5-7 lb base, and it really too big for that even with a weeks food.
For me, a huge load is 30 lbs. I used a circuit at philmont this summer and hit mid 30s once with 6L water. Rest of my gear is compact and light so I never had any volume issues.
I will be using Circuits on the JMT next summer. If there was a better pack to replace them with, I would have. I did consider a cuben circuit a few months back but havent pulled the trigger on that yet. Ive also considered the arc blast, but there are a few things that bother me about it still.