People in every sector of the web development community have heard of or use Google Analytics. It's a full fledged, corporate-level web traffic analyzer. It gives great insight into your content popularity, visitor characteristics, and more, and has some very cool (and easy to setup) reporting abilities. Being a fan of the open-source world and a generally "stay away from the man" minded user, I'm always trying out low-cost / no-cost alternatives to the big guns. In this case Google Analytics is free, but.. still. They are a giant.

Anyways, I think Piwik is a great alternative and has a fantastic, intuitive and very clever user interface. It does great tracking, has a ton of features that you'd find in something like Google Analytics, and it's also free. What are the downsides? Well, it's understandably in Beta still, but I've had Piwik installations that worked great for months and then stopped due to a MySQL insert hiccup. I've also had PHP warning errors come and go. These things are fixed in subsequent releases, but I can't stall a client's website tracking due to unfinished software. Sure, it's a bad idea to use beta software on a client's site in the first place, but regardless of the application usage, I think there's a more fundamental concept to think about.

Google Analytics stores information *off* your server, Piwik stores it on your server.

Sure, you could use an alternate server to run Piwik. Maybe that's even a recommended method. But the scripts that execute on page loads which store visitor information would be something you control, in your own domain (so to speak), and I've been leaning towards the "track your visitors from outside your realm"... realm. Store the information outside your servers.

Use a different, consistent, high-uptime server (like Google Analytics) that has its own team of developers that keep things running. If for some reason your own self-proprietary tracking system goes down you won't know until you've already lost real-time tracking data. It's the same methodology of using a website uptime service (like host-tracker). You shouldn't run scheduled tasks (or whatever method) from other webservers or your own PC to test your site's uptime; that opens the door to error. Use a service that helps you avoid the single point of failure. Just my two cents.