I haven't implemented it on any high-profile sites, but the few blogs I've turned it on for have seen anywhere between a 95-100% drop in their commentspam count, and the sites seem faster, but we haven't measured, and the traffic generally hasn't been high enough to serve as a real test.
Still, it's worth signing up for a free account if you have any sort of self-hosted site or blog.
LulzSec had their website behind Cloudflare's reverse proxy protective system ...
My experience so far is that it has made things faster. However, according to BrowserMob monitoring there have been several times during 1 day where my site is completely unavail.
So YMMV.
I use CloudFlare and it has improved my site performance tremendously and their DNS manager is awesome too.