I tried to use Piwik for about 8 months two years ago, on both a custom server setup, and on a recommended hosting setup at Arvixe, and there was just too much data for it to handle just one of my big media sites unless I purged everything every 2 weeks, which entirely defeated the purpose of having the historical visitor data that I wanted.

I spent weeks importing several hundred gig of raw http logs for it to crunch the data, so I could compare the numbers in Piwik to what GA was showing me, and while I loved the potential in it, I was unable to keep using it... the hosting company kept dinging me for database and resource usage, and their asynchronous tracking was actually slowing down my website's response time.

I hated doing it, but I gave up and removed it and imploded the Piwik setup, canceled my account at Arvixe, and have since gone back to just GA. It was a fun test, I just wish I could have duplicated what other people said, that they were able to track sites with tens of thousands of visitors per day with no problem. I wasn't able to.

If they've fixed the resource gobbling issues it might be worth playing around with again, but it won't be very high on my to-do list at the moment :)