Updated 2013-03-23: Note that this was originally posted on the Posterous service. Keep this in mind when reading the last paragraph.
Yesterday I got my invites for Flattr and today set things up (create account, put in funds, learn what it takes to get flattered).
Not too many sites have flattr buttons yet, but surprisingly many which I use. I now need to make a mental note to actually go to certain sites which have the buttons only on the web page but not in the RSS feed. We'll see how this turns out.
On the receiving side, things are more difficult — I added a static button for the whole blog. Adding per-entry buttons seems impossible unless you add them manually to each entry. I contacted posterous is they could make some tags for the templating system.