When did you decide to use a CDN?
Early on into development. The website is for a Photographer and one of the most challenging elements was making sure the load times were practically invisible. Whilst not as much of a problem as video, the simultaneous HTTP request limit was going to cause problems. The website is being developed used Google App Engine, and although they offer static content hosting there were problems with the number of simultaneous requests.
How did you measure the “success” of using a CDN?
1) Being easily integrated into the CMS so as to be invisible. This caused major technical issues on the server side, deferrable jobs and long AJAX async requests plagued - in the end the CMS workflow had to be reworked (and continues to be reworked).
2) Fast. Using Amazon's Cloudfront we have noticed huge speed increases, especially in the Orient (where the photographer in question will be moving to). File upload operations are also quick, however we are less than happy with the speed of META data changes (some can be quite extensive), especially on the S3 to CloudFront propagation.
3) Cheap. The cost of a CDN should be minimal for our needs and offer lots of bang for your buck. We were aiming for faster end user speeds than using Google App Engine alone, but with the same pricing and am happy to report this has been achieved.