I paid $50 at launch, and my account was extended when the cost dropped. That was money well spent; I expect to subscribe as long as they are in business. I love App.net because ...
- It has a very robust ecosystem of tools and services including multiple Mac and iOS clients and multiple web apps. I use Wedge, Netbot, NoodleApp and Appnetizen and will soon try Felix. There are multiple integration points to my Pinboard feed, including IFTTT support. Most of my posts start with Reeder.app  then go to Pinboard and turn into App.net posts via IFTTT.
- The community I interact with on app.net is exceptional. More on that below.
- I love the mission: a public (pay) communications infrastructure and related services that I purchase. I love paying for things I use.
- The app.net development team is delightful. I mean that literally; it's a joy to see them play and build on the platform - like @duerig's early stage Google Reader Share alternative, Patter-app rooms and private messages (EdChat) and vidcast shared video commentary.
- 128 characters is stupid. 256 is not twice as good, it's eight times better. (Though URL characters count, so I use URL shortener services)
- I'm 50+ and this is a relatively young community (though plenty of 40+ too). I'm old enough to enjoy that. The only young people I otherwise interact with are my kids (10-15) and their friends.
- I have the (illusion) of helping build something good without, you know, actually having to do anything. (Hence the illusion bit.)
- No advertising. Of course that doesn't mean no marketing; it means I choose the marketing I want)
 Alas Reeder.app may have been sunset. It's very unstable on the iPhone 5. Fortunately there are alternatives I can explore.
- The Economist's Babbage on two of my favorites: Pinboard and App.net. The Economist doesn't use bylines, but I suspect this is by @glennf.