We recently switched to MaxCDN for serving up the static content (css, js, image files) for our websites - roughly a million requests/day. MaxCDN is what we've always wished others would be like, but they never were. The MaxCDN network just works, serving up our content very fast and reliably. And unlike others, they have a clear cache button for when you need to use it! That means you don't need to keep your expire time set to 24 hours for changes to show up fast. And since MaxCDN offers a pull-type CDN config, there's no need to re-upload your content to a different place. Your files are on your regular web server, and MaxCDN caches them direct from there. Highly recommended.