It's weird, they claim that once you it a certain level they require you to either use Adwords or pay (I think you have the option of paying) but they don't seem to enforce it, do you happen to know if it's true?
The claim is 5 million pageviews per month if you're not an AdWords customer, but its not actively enforced, though all indications are that they'll get more serious about this over time.
No idea. I've definitely looked at MixPanel and would love to give it a shot, but my site would instantly put me into the paid range, which means I'll stick with GA for now.