Full disclosure: Keen is a promoted service on StackShare, StackShare is a customer of Keen.

We use Keen IO for all our user metrics. Specifically their CommonWeb library https://github.com/keen/common-web, which automatically tracks clicks, pageviews, form submits and a few other actions. Keen has been great for us, particularly because we don't yet know exactly what we want to track, it's constantly changing. So having one place that houses all our events then being able to query that as we see fit and build dashboards (http://keen.github.io/dashboards/) has been really helpful. You can see some more feedback from other people here: http://stackshare.io/keen-io.