I'd say that the biggest issue and the only issue I have with heroku is that when you guys have done some releases, you would prevent apps that have been placed into maintenance mode during the release from being taken out of maintenance mode.

This becomes a problem when the maintainers of the app in question are not aware of your release schedule. The solution, it seems, would be to either a) do a better job of communicating downtime; b) do rolling deployments and failover so that apps may continue to function; c) a bit of both.

While our app is not important enough for the above to bother us a great deal, we do have paying customers. As soon as one of them comes knocking telling us that we screwed up, and in reality it wasn't fully our fault, we will be a bit more adamant about the above suggestions :)

For the time being, great job!