Usually the biggest determining factor if a CDN is worth it is the distribution of your user base. If all of your users are within ~1000 miles of your server then it's probably not worth it. If your user base is global then it is.
as Pat said the main factor should be : do you have users from all around the country and the world.
recently worked with a major company that get 1/3 of their traffic from the west coast. their datacenters are on the east coast. we saw an amazing performance gain from that part of the country (2 seconds improvement). we measured both using our nodes and nodes on the last mile.