when do you add a cdn for static assets

We've been seeing a lot of traffic hitting our static assets lately, mostly images and JS bundles. Right now, they're served straight from our application servers, and that's starting to add latency for users who are far from our data centers. I'm thinking about introducing a CDN, but I want to make sure we're not jumping the gun. What's your experience with this? When do you find a CDN becomes a clear win for performance and cost?