Excellent work! Waiting for 1.6. I'm gonna give you a big donation when you upgrade :-)
There are many Options in the backend to customize. And if there's anything that doesn't have an option, it's just as easy to customize the themes as it is to customize joomla article-views. There are many great modules that come with the component - just as featured articles-slider e.t.c.
After installing (J15) everything worked very fine - only the tag-feature didn't. So I contacted the support and they immediandly responsed to me (on a sunday petecust!) and worked on solving the issue. A great support like that is rare and great.
highly recommended as there is no comparative blog-system available for Joomla
PS: A future-request would be that one can add multiple blogs (for example different blogs for different languages and different categories and conent).
Also there could be more languages available or at least a backend-language-file-access as I couldn't find a german translation anywhere.
Thank you! Flattr is something new and we're actually looking at their API :)
I asked the support, where I could add a div to put the whole comments-block in one Container (for some reason), but they said they wouldn't give support in that case, because even for them it's quite a bit work to do such a "small" change to the extension. But the Support responded very quickly anyway.
All in all a good extension, I would recommend, but not for someone who want's to customize form-layouts, xhtml-structure and so on... A trial version without license would be a good thing.