In my opinion, whether one button is “right” (or N-buttons) depends on how well thought the interaction with the software is designed. The hardware should match the abilities of the software, otherwise things start to get hairy and the hardware controls (ab)used for additional/unplanned purposes. One good example of ”no buttons can be good” (not even one!) would be the Nokia N9: swiping through the UI feels so natural that I still don't know anyone using it who would miss the buttons. The whole UI can be controlled flawlessly without any button at all. The power button has only one function (poweron/poweroff) and the volume rocker is just a nicety that allows for changing the volume without powering up the screen (good for people like me using it as music player), but still it is comfortable if it is not taken into use.