Amazon EC2 vs Dedicated Servers

We currently run 4 dedicated web servers and are thinking of migrating to Amazon EC2. What are the benefits vs drawbacks - in terms of costs and performance. We use 2 of our servers for serving about 3 to 5 million unique visitors a month, and 2 of the servers as database servers. We do not really need scalability benefits as we are happy with performance. We are worried that our performance - in terms of disk I/O and latency may suffer if we move to Amazon. Also, our costs may be higher. Any ideas or insights would be appreciated,