We run a live video cluster on EC2 that scaled from 10 nodes to 100+ and back again several times. Amazon keeps things running smoothly even when we're pushing close to 2gbps of video to 14,000 simultaneous clients. So I would say it performs very well for jobs requiring lots of computation or bandwidth.

However... we have our web servers in a colo. Ping times to EC2 aren't fantastic, so we use a traditional CDN for static content and a colo for application servers in order to keep things snappy.