Well, I went and installed piwik on all of my sites. Between awstats, Google Analytices, and slimstats I figured one more package doesn’t matter…
Piwik is clearly mimicking GA’s look and feel. It doesn’t have all of GA’s features, but that’s not necessarily a bad thing. I have minor issues with the user interface, but nothing I couldn’t live without. I like it very much that I own and control the data, as opposed to Google. I dislike the way piwik is instrumented; I’d rather have a PHP include in index.php like slimstats rather than the JS solution both piwik and GA use. I haven’t looked at their plugin API and how easy it is to write your own plugins, but at least the option is there.
All things considered, I’m still looking for a one-stop analytics package. I like to look at awstats for a mile-high view; slimstats (or mint, if you want to pay for it) is good to display real-time activity, GA and to a lesser degree piwik are good for a more elaborate analysis. GA is overkill for my needs and I have trust issues; sooner or later I may ditch GA in favor piwik.
As always, YMMV. Whatever floats your boat, eh.