Do you have had any troubles? How does it scale? We have a large web site with several million page views per day. Are you satisfied with the reports? Do you run it at a dedicated server or the same server your web site is on? Are there any serious bugs, is the software supported and updated regularly?
Did someone of you switch from Google to Piwik and was satisfied with the change?
I had it a long time back but I still prefer Google Analytics. Last year, their site got hacked and some malicious code was inserted into the installation files. Some sites were affected. I don't see anything good about Piwik other than the fact that it's open source. For me, I never needed an open source alternative to Google analytics so I removed it at a later time.