GameSpy have posted a review of the new Xbox Controller S. It got a very favorable review. Here is part of it for your viewing pleasure:

When you first take out the Xbox Controller S from its packaging you will notice how much smaller the controller really is than the Xbox pack-in controller. To give you an idea, the Xbox Controller S has roughly the same width as the Dual Shock 2 PS2 controller or the standard GameCube controller, but is slightly taller. The Xbox Controller S feels very comfortable in my hands, is less bulky, and is a lot lighter than the pack-in controller.

A new button layout is featured on the Xbox Controller S, with the cross-shaped layout for the main X,Y,A,B buttons reminiscent of the Dreamcast or SNES. The smaller black and white buttons, that were previously located right beside the main X,Y,A,B buttons, are now located just beneath them. The "Select" and "Back" buttons were removed from the traditional center spot of the controller, and relocated just under the left thumbstick beside the d-pad. The relocation of these buttons took a short while to get used to when testing out the Xbox Controller S, but because they are kept beneath and to the side of the main analog stick, they don't get in the way while using the controller.