I use cloudflare on mainstream sites and love it. They were responsible for mitigating ddos attacks against lulzsec. I use it for spam prevention and it is pretty good for that. Page load times improved about 30%. Try it on a smaller site first and see what you think. Everyone I have sent to CloudFlare raves about them.
lulzsec/cloudflare article -
http://allthingsd.com/20110610/web-s...lzsec-hackers/