As far as CDNs go, I think CloudFlare has been fantastic. Trivial to use, as well.
Just last week, their anycast servers in Chicago blocked all traffic and said my server was down even though it wasn't.
It took them about 3 days to fix it, so I am a bit skeptical about using them in a production environment.
It's also free - and besides making your site faster, also blocks a lot of the spam you would normally see.