Recently we decided to compare CDNs. Using NewRelic we setup a test on Synthetics. Pretty interesting data.
Edgecast All counties
Edgecast Just San Francisco
EdgeCast seems to have occasional long hangs even in San Francisco.
S3 Just San Francisco
S3 seems to be slow, but very consistant
CloudFront Just San Francisco
Cloudfront has the greatest speed bursts, however, suffers from a few long pulls.