http://xbox360.ign.com/articles/652/652597p1.html
I'm really looking forward to playing games on this controller...
The main and most important thing about the design is that unlike the Xbox 360 controller, which has a slightly rougher surface texture than the smooth-surfaced Xbox 360 controller, is that it's designed to fit in your hand comfortably. It is thinner and less bulky underneath the central section. It's also thinned out around the inside contours of the "handles" -- the prongs on each side that you grab onto. It's crafted to rest in your hands with comfort and usability in mind.
The black and white buttons, now moved into the shoulder positions, are also petite. The base of the controller subtly protrudes out like an outline and the shoulder buttons push out through them. The shoulder buttons feel and handle perfectly, and depress only a small fraction -- less than the PlayStation 2 shoulder buttons. This really doesn't amount to much in terms of gameplay, because the response is still instant, but it does mean more from an ergonomic standpoint. The buttons don't require the fingers to extend as far, making the feel of the controller, again, more streamlined, comfortable, and ergonomic.
