I recently picked one up on sale but the waist belt didn't suit me, so i returned it.
According to the MR sizing guide i was between Large and Medium and their recommendation was to size down so i went with the Medium. There is a rigid "brace" that comes off the bottom of the frame and feeds into the waist belt. I'm assuming this design is to stabilize the load laterally like when side-hilling. But in my experience it dug into my kidney area and was quite uncomfortable from the start. I can only imagine how much worse it would be after many miles with a load.
I'm not sure if a Large would have made it better, but ultimately i decided i preferred a dedicated external frame pack where i could switch out bags for day hunts and back country, so im back on the market.
Everything else about the popup was great though. i liked the bag and the popup concept is really cool.