I used Chartbeat on Hackerbuddy.com - it has customisable alerts which can go straight to your iPhone, so when traffic spiked I was able to react. The app hit the front page of Hacker News, and I was able to join in the conversation early on, which was really useful. It also got mentioned by Mashable, and Chartbeat alerted me and let me share the Mashable link around. It also monitors uptime and alerts me if the site is down - which means I get to reboot the server pretty much as soon as it happens. I'm not 100% sure real-time analytics is a requirement for everyone, but for me it's been extremely useful.