The software is still in early beta, but looks very promising. The advantages are clear, you have full control over your data and don't have to share them with google (don't be evil - I know
I already use the tool for about 5 days. A longer usage was reported in the German section of this board (Thread here) with a live view to the statistics of a board member.
I think it could be even possible to tightly integrate piwik as a plugin into serendipity, as it was already done with some other systems like dokuwiki. Maybe this could be a nice feature for a future version of S9Y?