Well, I can do all the speed improvement things myself, so I'm interested in some of the weirder things. They can detect bots quite reliably because with a network that large it's pretty damn obvious when something is crawling a site - and they can then strip out or mangle email addresses from your HTML before reaching those bots, so you can have email addresses in plaintext without having them harvested. That's a pretty imaginative solution!
Another thing they can do with their size is automatically identify and react to new attacks, so when a DDOS occurs it very quickly routes the traffic away and updates its ruleset to always be protected against that attack from now on, for all sites behind them.
I don't actually have any experience though as I've never used the service, this is all from their website and from talking to their staff on HN :)
CloudFlare does proxying and cashing for static and dynamic pages too and adds some security stuff. Not sure they minify and compress static resources, though. So, both services look like they are very different. CloudFlare will speed up a dynamic page that loads slowly, Google's service won't.
