I've owned all of the Zoom Flys, and this is by far the roomiest in the toe box. True to what you'd expect from the Zoom Fly, they're extremely comfortable. I run approximately 30 miles per week, and I use this shoe for the shorter distances. Unlike its predecessors, this shoe feels "clunky" after six miles or so. It's not heavier than the Zoom Fly 3, so the only thing I can attribute this to is the toe box area. Other than that, I highly recommend the shoe!