I haven't tried Amazon EC2 in production, but I understand the appeal of it. My main issue with EC2 is that while it does provide a great and affordable way to move all the blinking lights in your server room to the cloud, they don't provide you with a higher level architecture to scale your application as demand increases. That is all left to you to figure out on your own.
This is not an issue for more experienced shops that can maintain all the needed infrastructure by themselves, but I think smaller shops are better served by something more along the lines of Microsoft's Azure or Google's AppEngine: Platforms that enforce constraints on your architecture in return for one-click scalability when you need it.
And I think the importance of quality support cannot be underestimated. Look at the BitBucket blog. It seems that for a while there every other post was about the downtime they had and the long hours it took for Amazon to get back to them with a resolution to their issues.
Compare that to Github, which uses the Rackspace cloud hosting service. I don't use Github, but I understand that they also have their share of downtime. Yet it doesn't seem that any of that downtime is attributed to Rackspace's slow customer support.