Recs.
Updated
CloudFlare is a content delivery network and distributed domain name server service that improves website performance and speed and provides security.
Specs
Pros
Pro Handled some of the largest DDoS attacks in history
Cloudflare was able to deflect 2 massive DDoS attacks. During the March 2013 attack on Spamhaus, they were able to absorb a peak 120Gbps attack that lasted 4 days, as well as a 400Gbps attack in February 2014.
Their track record shows their ability to protect against DDoS attacks in practice.
Cons
Con Practices Man-in-the-Middle certificate forgery
Https ("secure") comunications with sites using CloudFlare are intercepted at their servers, decrypted and recrypted with CloudFlare's certificates. This poses huge problem with what users perceive as safe communication - browsers fail to display notice about MitM taking place.
Con Support doesn't have access to logs
Log access is an enterprise feature and priced at the "contact us" level. So when an error code is returned to the user that wasn't returned from your app, debugging this is impossible at the pro level. Unfortunately these logs aren't available to support personnel either so they have no way of tracking/validating issues.