I love Amazon Prime, being a student I got it for $39.99/year, and with my wife having the Kindle on my account it's nice getting the book a month. I'm with Kat though, they really should let you do it like once every two weeks or something, that is a bit weak, but free is free...
Also I should mention that sometimes with the free 2 day shipping they will give you a bonus to downgrade to regular shipping (which is still free), like a dollar or two worth of MP3s or videos, etc. Not a huge thing, but hey I got a few dollars to spend on a video while doing Christmas shopping.
I don't know about streaming ALL the episodes they have for purchase, but there are plenty. Though it seems to me a lot of what Netflix has, they have, so if you have Netflix there is a lot of doubling up on shows. If you don't it's no different than a Netflix subscription per month, and you get the free shipping and other perks...