In a nutshell AMA membership gives you liability insurance if you fly with the site owners permission and follow the fairly simple safety code. In spite of what some people think it applies even at non ama clubs if you follow the above 2 steps. It also comes with a magazine subscription and allows you to fly at sactioned events or fields if you are traveling or whatever.

For the cost of the insurance alone its worth the 58/year in todays law suit happy society. The magazine has a little bit of something for everyone as well.