setzer - 8:08 pm on Sep 23, 2012 (gmt 0)
I've used some top tier CDNs in the past like Akamai and EdgeCast on a fairly big site. No change in ranking that I could attribute to the CDN usage.
A CDN is only really needed if you want better response times in regions that your main server is not located in. When we used Akamai, our US visitors didn't notice a difference in speed compared to serving content directly from our Softlayer server. Buying a CDN to improve Google rankings is a bit misguided I think, I've seen a few CDN providers advertise this as a bullet point but there are other ways to improve site speed that have a far more measurable impact. Consolidating small images into a single image using CSS sprites is a biggie.