This is my point of view, as a long time CDN customer who's also been through countless hosting companies:
We've been using CDN for over 3 years now. We tested several providers (HighWinds, EdgeCast, Level3, 3Crowd, LimeLight and a couple others I'm forgetting) using A/B tests and measuring latency, download speed and Up-time for small objects (images) and large objects (videos).
It turned out that overall, Level3 (who provide the CDN for resellers like Reflected/Swiftwill and others), is the best solution. They provide the bandwidth for NetFlix so their infrastructure is robust, their prices are unbeatable if you use a reseller with enough leverage, and all the features you can imagine are already built into their system.
Regardless of who you go with (a reseller or Level3 directly), I think it's a good choice ;)
And after having tested 2 separate Level3 resellers, I can say that Reflected.net has the best support. Their techs are accountable, they reply to tickets promptly, and their integration of the CDN is very impressive (the admin has graphs per location, bandwidth per domain, file invalidation, config, etc). Plus, if ever their techs make a mistake, not only do they own up to it immediately but you'll enjoy a credit rather than be given the figurative "talk to the hand" response.
There are other companies that offer packet optimization and/or other types of non-CDN page load "accelerators" but if you have enough bandwidth, the beauty of CDN is that you can push several hundred Mbps, even multiple Gbps using a single origin server (lower server purchase/rental fees, near infinite scalability, etc).
