I tried CF on 3 of my domains and it works really well! It not only speeds up your website (significantly) by doing some caching operations "upstream" (they also have a server in Paris :) ), it also enhance security.
From a FAQ:
CloudFlare is a broad security solution that is designed to provide protection from many forms of malicious activity online including: comment spam, email harvesting, credential hacking, web software vulnerability, cross-site scripting (XSS) and SQL injection attacks. Both XSS and SQL injection attack protection are part of the Pro service.