We use it in our product. It's incredibly easy to set up, and the entire focus is on tracking events happening in your code.

I.e., we have an event for people pressing the login button, another event for people actually logging in, an event for posting a message, etc. Very simple to set up, and now gives us real-time analytics (and funnels) on how many people press login vs. actually login, etc.

And those are just small examples - we have a huge range of events that we track, things we care about.