As little as I've played with it on SC, the Hitbox already feels GREAT for 3D fighters. Stepping is such a breeze now, since I've always had problems double-tapping up naturally on a stick (this could just be a 'me' problem).
Anyway, like most of you, I did have problems and initial skepticism about the up button being more of a spacebar position rather than a WASD position. After playing with it for 2+ months though, I have to say I can't imagine it being in any other place. This placement obviously has more benefits for 2D fighters given tiger knees, but I see how this set up makes it much easier to switch between up and down at any given moment. This is because just rotating your hand will make you go from up to down, rather than taking your finger off the up button to go onto the down button (if this makes sense).
But, like I keep saying to everyone, if you want to learn to play on a Hitbox, the Up button will screw you for a while. :p
I do get asked this question a lot (especially my SC contacts) so I might do a video tutorial just for 3d fighters, since the HB guys have been solely focusing on 2d fighters because of current game popularity.
PS.
For those with really wide fingers (not hands, there's a difference), the current generation of the Hitbox may not be for you as the buttons are too small. The buttons are big enough for small-slightly big fingers, but maybe not more than that. They did make the box itself wider for those with good size hands, but kept each button size the same. (Just my two cents though -- I have wide hands but narrow fingers like piano hands).