The persistence scaling story for Heroku seems pretty questionable to me. Once you've maxed out what they offer for MySQL and Postgres, what exactly are you supposed to do? Start using EC2 RDS?
Heroku seems to be like a more useful Google App Engine, a good place to host a blog or experimental project if you're not into dev-ops.
If you have a knack (at all) for dev-ops, you're not saving yourself anything.
The downtime is pretty bleh too. The moment you start doing multi-provider to offset this, you'll end up doing all the dev-ops work you would've had to have done before. Except now, you have to do it all at once in a time of what is probably high stress.
If you do the dev-ops/automation yourself from the start, you can start small/simple and grow that as you go, deploying your services to arbitrary hosting providers (EC2, Linode, dedicated boxes, whatever).
This is why whenever anybody asks me my opinion of Heroku, I respond, "it's a great place to host that blog engine you wrote in Haskell/Clojure/{hipster_language_of_choice}".