when's it time to think about a cdn

ive been working on a web app with a global user base, and lately, load times have been creeping up for folks far from our servers. we're serving a good amount of static assets and some dynamic content that could potentially be cached. im wondering at what point you all start considering a Content Delivery Network (CDN)? is it purely based on traffic volume, geographical distribution, or are there other metrics you look at before making that jump? whats your experience been with implementing CDNs for the first time?