Thats exactly how I feel as well. I would pay for a social network if it created a new/better experience for me, but App.net is a twitter clone. What can I do on app.net that I cant do on twitter? What part of the experience do they beat Twitter on?
App.net also hasnt been successful enough to attract the majority of people I follow on Twitter. So their value proposition for me looks like this: Im paying for something that looks, functions, and feels like Twitter without the people to follow to create that content.