When do you actually need a cdn for static stuff

hey every1 im workin on a project with a lot of static assets like images, css, and js and were starting to see some page load times creep up, esp for users far away from our servers. were thinkin about gettin a cdn to cache these assets closer to ppl. any1 here done a cdn for stuff like this? what was the hardest part and what was good about it? any specific metrics i shud keep an eye on after we set it up