One of the things that I found while working with GamePadShift is that reading it too fast will sometimes return 0 even if there are buttons pressed. I believe this is because it shifts the bits out when it is read and doesn't maintain state. I'm not sure if this was the intended behavior or just a result of a fast CPU reading faster than the shift register is reading the buttons. I think it would be useful to maintain the state after reading the buttons so subsequent reads don't show up as no buttons being pressed.