I have it, and use it quite a bit. But I also buy some things that i could get in the store, from there, generally if i can find it for less, for the convenience. I've also bought clothes and shoes, even if available from other sites, because for a lot of it, they offer not only free shipping, but free returns. So for me, its totally worth it.
I also take advantage of the streaming video. The free monthly book? Honestly, I've found very little that interests me. It's never big name authors or bestsellers. But since they sell pretty much everything under the sun, I take advantage and buy a lot, since its free to ship, and comes quickly.