Mixpanel was extremely useful just to see where the huge holes in our funnel were. For example, only 30% of the people that see our signup page end up signing up. Now that we know a lot of people are following through with signup, we're focusing attention this week on improving that number.

We just started using Optimizely and so far it's been great but I haven't used it enough to really comment. Optimizely seems to be really great for micro-optimizations and we've been focusing on bigger complete page redesigns thus far.