The owners of flattr also own thepiratebay. I remember hearing a speech a few months ago talking about how everything should be free, just like when we were children (we need to learn to share). They have no regard for the rights of developers and have no problems giving a clear and direct roadmap to a free copy of your commercial application.
If they really believed in sharing, freedom, and rainbows, why does Flattr charge a 10% surcharge on top of every transaction (more than even Paypal). I might use them if they didn't make a profit and only paid for the infrastructure and bandwidth of the service.
It seems most people here don't see them for what they really are: a bunch of greedy fucks.