I don't have personal experience with Heroku or EC2.
My feeling is that, coming from my background (system engineer: I can do basic Linux administration but I don't breathe it) and at my scaling requirements, EC2 is more complexity versus "as simple as dedicated" VPS providers like Slicehost without any compensating advantage. Heroku seems like it would potentially be amazingly useful for folks who swing more programmer than systems engineer, and quite possibly for me as well, but I just have never gotten around to trying it out for a test drive.
My feeling is that, coming from my background (system engineer: I can do basic Linux administration but I don't breathe it) and at my scaling requirements, EC2 is more complexity versus "as simple as dedicated" VPS providers like Slicehost without any compensating advantage. Heroku seems like it would potentially be amazingly useful for folks who swing more programmer than systems engineer, and quite possibly for me as well, but I just have never gotten around to trying it out for a test drive.