I'm migrating my websites from Google Analytics to Piwik and thought I would share my experience, in case its relevant to ThinkLab or any ThinkLab users.
Google Analytics is the most popular platform used by webmasters to analyze traffic to their sites. However, since it's a closed source and third party solution, the feature set is predetermined/inflexible and users must rely on Google to provide the service and manage user data. The advantages are easy setup and an impressive feature set that integrates with the larger Google knowledgebase.
Piwik is an open source analytics platform. Piwik can provide more detailed user information, such as IP addresses, and is extensible and modifiable. If you host your own Piwik instance on a server you control, user data is not shared with a third party and there are fewer potentially problematic dependencies (for example, some counties block Google services). The downside is greater setup, maintenance, and cost compared to Google Analytics, as well as lack of integration to the Google knowledgebase. The main benefits are complete control, data ownership, and user privacy.
Regarding user privacy, Piwik may not necessarily be preferable to Google Analytics. For example, Google's expertise and size could allow them to implement superior penetration resistance than a single webmaster. The privacy and security issue centers around whether Google or a website's private webmaster is more likely to accidentally leak or misuse user data. This is a complex issue and likely situation dependent.
My Personal Experience
Personally, I have mostly migrated my sites to Piwik from Google Analytics and prefer the interface and additional details. I used the Bitnami Image to quickly get Piwik running on an Amazon Web Services' EC2 Cloud instance. To get a free SSL certificate to secure communications with my AWS instance, I used StartSSL.
ThinkLab currently uses Google Analytics, along with most other sites I've investigated. Given the complex set of drawbacks and advantages, it's difficult to say which service is superior, but I wanted to throw out the idea of Piwik.