I flew the 321Neo for the first time a few months ago in J, and while the rest of the experience was positive, I still find it really odd, and irksome frankly, that the underseat storage is SO shallow vertically. I fly the E-175 frequently and my Euro-sized and not-overstuffed backpack fits perfectly but on the Neo it had to go in the overhead. I talked with the FA about it and she too was surprised the first time she was a passenger and had to stow hers in the overhead.