Here using Fastly with great results. Consistently fast, including SSL negotiation. Truly fast invalidation. A ton of control available with Custom VCLs (they run a custom Varnish version). E.g. you can generate an "Expires:" header automatically on the CDN edge based on the Cache-Control header. You can serve a static resource if the backend times out. Best of all
stale refresh and soft purge means that you can update resources "in the background" on the CDN and avoid exposing your clients to "miss latency" (backend server latency).
I feel that the ultimate answer is a multi-CDN solution. If your assets are critical to be always available, I think you are going to want to diversify your CDNs. The challenge comes in how to load balance and/or failover across CDNs--whether that's easy or not depends on your use case. Doing so automatically at the time of trouble is a challenge too. Dyn just rolled out a product called Internet Intelligence that monitors all the CDN's performances (with end-user perf measurements) and can help you determine whether one of your CDNs is slow (and whether it's slow only in certain parts of the world). Theoretically, then you can combine geo-aware DNS with congestion-aware CDN selection and BAM, diversified, less-congestion-sensitive content delivery.