I've run http://Dibs.net on EC2 for over a year, with images served up on S3. Several servers have uptimes since I booted them in June '07. I have nothing but good things to say about my experience with AWS.

If you can, I highly recommend going to the AWS Start Up Tour (http://aws.typepad.com/aws/2008/08/2008-aws-start.html) It's a good place to meet people and last year Jeff Bezos and John Doerr showed up to the one in Palo Alto.

Edit: Forgot to mention - I run PostgreSQL on instance storage with log shipping and backups to S3, and a couple of MySQL DBs for Cacti and geolocation lookups. EBS will help a lot in this area, which is kind of a pain right now. Can't wait to start using it.