My xbox copy hasn't arrived yet (running frustratingly late that order is). But i to, do this with the ps3 controller. However although indeed possible on a regular controller, a hitbox gives you so much more precise control. You can't really punish with any efficiency doing this on a regular controller. I'd really like one of these. I've always felt like keyboards are actually superior in fighting games.
Also in my opinion, any peripheral that decreases input time is a good thing in any game. As long as it uses the same control scheme e.g. up/down, X amount of buttons (no macros) an so fourth then the legality of such a device should not even be up for discussion. That's how i feel. I realize this becomes troublesome for those who for whatever reason does not wish to use said device even though it's a better input device. It's the good ol' causal/hardcore debate, if a game requires learning to use a new 200$ controller to play competitively then of course player base will decline. Also we already have fighting sticks. They're of course legal, even though they offer a major advantage over regular joypads.