We use it here at Optimizely and have been very happy. It hasn't been smooth sailing all the time, and we've gradually grown our stack to include EC2, S3, and several CDNs as well, but overall we're very happy with it and have no plans to leave.

The biggest benefits for us are the scalability (huge), ease of deployment, and the monitoring. The biggest cons have been lack of support for SSL on custom domains (HUGE hassle) and lack of official support.