It's true! This bag does fit underneath the seat with room to spare.  Multiple pockets and the detachable 3-1 bag is a nice option.  A small water bottle can fit in the side pockets, but it is a tight fit.  I wished they would have included some mesh there, so it would expand more to accommodate a larger water bottle instead of the plastic, at least on one side.  It is a great place to put your boarding passes or travel documents though..  Most of the other reviews cover this bag thoroughly, but I hope the pictures will help you decide!  I flew on Delta  and sat in an aisle seat (3 seats on each side of the aisle), and the bag fit with at least 4-5 inches to the side.  I also flew on American-Embraer 170 jet (2 seats on each side of the aisle), and it fit just fine as well. This bag will relieve your stress of fighting for overhead bin space!