I've been extremely happy with CloudFlare. 4chan probably wouldn't be around today without them. CF has helped us by mitigating every DDoS since we started with them two years ago, and have kept operating costs in check by offering flat pricing. Even at their highest tier (Enterprise), we're saving money versus what we'd be spending with our colo.
Another point for Cloudflare here. Seriously boost in site responsiveness after we went with them on our site. They also have a CDN of JS and CSS libs that we use extensively at http://cdnjs.com/
Just adding my personal experience: my sites went from a 30ms response time (measured by Google's webmaster tools, as Googlebot sees it) to over 250ms on CloudFlare, for static (cacheable) content.
Turning it back off gave a very clear improvement both in my own experience and according to Google's chart.
