We use MaxCDN (NetDNA) at WPForce.com, and at $40/year for 1tb of bandwidth, it's tough to go wrong. I find it's super speedy and the interface is one of the best I've seen for a CDN. Plus, the management team are very friendly and easy to approach.

I've tested out Photon, but I found having an actual CDN, even if it costs a little bit, is worth it.