Hm... we're happy users of both and they have almost nothing in common. Mixpanel is more like a flexible general purpose database which gives you real time "views" of your data. You can chart it yourself or Mixpanel will do it for you.

We (http://mailgun.net) built our email analytics on top if it in a very short period of time and it's much nicer that anything we could have made in-house (and keeps getting nicer).