We started off on Heroku because we wanted something dead simple. The amount of time heroku saved us was an incredible value when we were starting out. I don't think there is any vendor lock in. We just did not switch to AWS earlier because our needs were met really well by Heroku and even Amazon Elastic Beanstalk (for ruby) did not come near the ease of a Heroku deployment. Once Opsworks came around, we invested in deployment scripts and switched because Opsworks gives us same ease of use and greater control of our stack.

What do you think stops you from switching? We had no issues at all - definitely none from Heroku.

We still use PostgresSQL from heroku because it is still a solid service and comes with niceties like dataclips. I should confess that I have not explored the Amazon PostgreSQL offering but I am happy with Heroku for databases at the moment.