Because Cloudflare acts as a proxy, it gets in your way in subtle but devastating ways. First, the SSL support wasn't stable and I turned it off a few days into launch. That probably killed some traffic.
Then, I realized that caching was an all-or-nothing proposition. You can have Cloudflare cache your assets, but it doesn't seem to respect expires headers. You can set a cache time in the web interface, but the minimum is 2 hours! So you end up creating a rule to have it cache no assets, at which point you might as well use AWS Cloudfront for a CDN.
I advise you not to use Cloudflare. It's a great idea, but the execution just isn't there.