I am a sucker for Amazon. I have been using them for many years now, and they are also the bane of my existence because I already have too many books as it is (just ask SigO).
Then they come along with this Amazon Prime dealybob and make it even harder to resist. You had me at "Yes, we have a dazzling array of wares that you no longer have to drive all over the known universe to find in real stores."
It has already come in handy because weird shipping vagaries resulted in two separate shipments to the birthday boy this week, and I didn't have to pay for either of 'em. All in all, it really seems like a better deal for Amazon than it does for me. I'm incented to buy from them rather than their competitors, and I will most likely end up spending more with them in the long run.