Google Analytics has a delay. Mixpanel is real-time and specifically aimed at certain events that you specify. So when someone clicks your sign up button, it fires an AJAX call and you see them do it in real-time, instantly.
I like what MixPanel is doing, but I don't think it's fair to say that GA has a 24 hour delay. It's nothing close to real-time, but you can get results from the past few hours.
I'm looking at Google Analytics right now for my web site, but all I can see is yesterday's data. Is there somewhere to see the data collected today?
Edit: I use both Analytics and Mixpanel, they're both great services. Google has its strengths, but I don't think event tracking and immediate feedback is one of them, which is where Mixpanel comes in.
Click the date select stuff, you can choose today and it'll give you whatever data it's processed for today, but by default it only shows up to yesterday.
