so when do you actually need a cdn, like whens it a good idea to start using one.

So, I've been pondering content delivery networks recently. You know, since we have a pretty global user base, sometimes load times for static assets can really be a drag. I've seen CDNs brought up as a fix for that. What have your experiences been like? When in a project's life, or with how much traffic, do you usually start thinking about adding a CDN? Are there any particular numbers you watch before you decide to go for it?