I buy from Amazon out of convenience. It's not always easy in Boston to drive down the street to get what I need... and oftentimes, I don't need it right away. 2 days is reasonable for most purchases and Amazon delivers (pun intended).
Looking back at 2013, I made 72 Amazon Prime purchases. The prices on the goods were either the same or less than bricks and mortar on almost all of these items. I saved anywhere between $2 - $30. Let's say I only saved $2 on every item... that's $144 dollars saved and I didn't have to spend the time, energy and resources to go get the goods.
For me, it's worth the convenience. I don't expect to save, but I think I can at least break even.
I also share the account with my sister, so there are savings and benefits I haven't counted there either. Lastly, I sometimes make a purchase for a friend through Amazon Prime and get the benefit of earning rewards points.
Things add up and it makes sense to me.