We use CloudFlare as a CDN on Pokécharms, and the results have been great for us. Most of our savings come from attachments, as our main application generates and hosts small images, but we've got our page rules set up to cache as much as we can.
In terms of stats, we've got 1,983,463 cached requests vs 637,593 uncached requests in the last 24 hours, and that makes for a 75% reduction in requests, with a 72% bandwidth saving. (25GB cached bandwidth, 10GB uncached)
Settings-wise, we've got auto-minify turned on for HTML, CSS, and JS, to help smooth out any unoptimised files, and three page rules:
*pokecharms.com/css.php?css=* – to cache XenForo's CSS output
*pokecharms.com/proxy.php?image=* – to cache images in the image proxy
*pokecharms.com/data/* – to cache the assets used by our applications, and to cache our attachments (we use [bd] Attachment Store for our attachments)