When should you think about using a CDN for your app?

Hey everyone, I've been building some web apps lately, and CDNs keep coming up. Right now, we're mostly handling static assets, but I'm wondering when it really becomes a must-have instead of just a bonus. Is there a certain amount of traffic, or specific performance issues, that make you say, 'Okay, it's time for a CDN'? What do you guys think? I'm eager to hear what you've all found.