As much as I order from Amazon (and have for many years), Amazon Prime is no-brainer for me, even at twice the cost. I've been a subscriber pretty much since it was first offered. Not even counting the extras perks like streaming video, music, 5% cash back on my Amazon Prime card (which btw more than pays for my Prime membership), etc., just the free shipping on almost any order makes it totally worth it for me.
I have a bad habit of going shopping and forgetting one or two items. This is true with the supermarket, big box stores, or online. With Amazon, I try to buy what I need in one go, but always forget something. Having Prime, I don't think twice about ordering that small item I forgot, as soon as I realize I forgot it, rather than waiting until I have a bigger order to get free shipping. Prime is a good investment for someone like me.