Note that they don't cache anything over ~50mb, and it's a relatively small cache (~4GB nodes, cleared LRU first), in addition to only revalidating every 5 minutes.
Why don't forums like Hacker News, Reddit, etc. build this into their submission process, so the article/page is cached upon submission, and readers are directed straight to the cached copy?
http://wiki.coralcdn.org/faq.html
So don't use it for big files or anything that changes more than every 5 minutes.