Just got back from an abruptly ended elk hunt due to electrical issues on my truck. I have the 2200. The bag itself is bombproof, well-constructed and has a streamlined slim contour to it. I love the bag.
The one negative, which I have seen mentioned many a time before but hadn't experienced until this hunt was the design of the side zips. Why they were built to expand and rob space from the inside of the main compartment instead of expanding externally, not taking up any additional internal space is beyond me. I had my puffy pant jacket on one side, rain gear on the other. There might have been a 4" side to side opening to try and stuff down into the bottom of the bag below where the sides pockets terminated however the middle and upper third of the bag where the side bags encroached robbed me of valuable space.
I would 100% move up to a 3600. I didn't think I would need to for basecamp/truck hunting, but I would rather have more room and not need it, and cinch down the bag nice a small, than not have an option for more space.
The K4 frame.........AMAZING. So comfortable and stable. The side pockets on the 2200 are my only complaint.