Prime members...were inclined to spend more based on the "faintly irrational human impulse to maximize the benefits of a membership club one has already joined."
For some of us, the choice was completely rational. After noticing that at least a half-dozen times per year we were upgrading to 2 day shipping (which is about $15 each time), it made sense to get Prime. It is even sweeter since we could cancel Netflix at the same time and save $96 a year there as well (though the selection is more limited on Amazon).