Amazon Prime is great if you buy a LOT of stuff but it's not good if your just making a couple dozen orders per year.
They make their money on the people who under utilise it, so make sure you are a heavy shopper before enlisting.
There's more to Amazon Prime than free shipping. My wife gets lots of streaming TV shows and movies, plus free books with Prime, on her Kindle fire. And I have the Amazon Instant Video app on an iPad that gives me the same goodies. Plus with the free 2-day shipping, it's a good deal for us.
But you're right, if you only use it for free shipping, and don't order much, it's not such a good deal. Though with high shipping costs for 2-day shipping, you don't really have to be a heavy shopper to recoup your $79.