MaxCDN is a really good value and the cheapest for an anycast cdn. vps.net resells akamai for $100/TB ($0.10/GB) which is also a great deal although you don't get netstorage with that (origin pull only). Simplecdn is not a great cdn, we've actually experienced outages on their service and performance generally sucks. cloudfront is good and getting better as they add more pops. If I were to pick a cdn I'd go with edgecast (gogrid or speedyrails resells), cachefly, internap (softlayer resells), cottendo.

> Also, why so many people using Cloudfront?

Path of least resistance :)