We have been using Amazon's EC2 service to host our Magento eCommerce website on Amazon's AMI. We have been doing this for 10 months now, and between uptime and manageability we are very pleased. We chose this platform for the attractive prospect of usage-based billing and potential for provisioning more resources as we grow as well as auto-scaling and load balancing options. The migration was hand-in-hand with a re-development, platform and host were new, so comparing to our old Wordpress store on shared hosting would not be apples to apples at all. All said, we have been please with AWS EC2 on the whole.