Was invited to the exclusive event to get to see and demo the preview of Xbox Kinect. So far the motion capture was pretty accurate from what I see. Accuracy wise is about 95%. It is able to detect the vital point of the human body except for some cases where clothes might be cut off a bit but it doesn't really affect the gameplay.
So far the impressive game which I see for now are, Your Shape Fitness Evolved by Ubisoft, Kinect Adventures by Microsoft and Dance Central by Harmonix.