This is like saying, "Famous actors frequent my restaurant" and point to a picture of Ralph Macchio. OK, somewhat known, but De Niro, Brad Pitt, Clooney and co eat at the joint across the street.
Once you've worked out you have a product that people want you can start thinking about managing your own servers.
People complain about migration, but its really not that big a deal.
Does Heroku have any particularly high-traffic sites?
Urban Dictionary, as an example.
Here's another one I haven't heard of before now:
1. Migrate away from Google-hosted App Engine to your own AppScale instances.
2. Negotiate a better rate with Google.
In my experience App Engine has been fine for apps that only get a few hundred or thousand visits per day. The convenience of not worrying about the environment or deployment too much is a great enabler.
It definitely depends on the type of app. You probably wouldn't want to run anything that is particularly heavy on datastore use. Using AJAX rather than full pageloads is a good way to keep bandwidth and response time (and therefore instance hours) down. For example, one of my apps is able to pre-load the most important AJAX calls, so it can handle running with high latency and I don't exceed the free instance hours.
We pay under a hundred dollars a month for extremely reliable and scalable infrastructure. I'm happy that our platform is designed and supported by the best sysadmins in the world - we've had less than five minutes of degraded service (high latency but still accessible) in the last six months. I'm soooooooooo glad we didn't choose AWS!
Appengine is perfect for developer-centric startups. You can concentrate on your app instead of the infrastructure until you've got traction, and if you do hit the big time you'll be popping champagne corks instead of server cores. At that stage you'll be able to make an informed decision about hiring some kick-ass sysadmins to build your own platform, and migration really isn't as big a deal as some people make out. Python apps are WSGI compliant so will run with minimal modification on any WSGI server, and you can access all your data without difficulty. You would need to write some wrappers around API calls, but this isn't a huge deal and realistically you're going to have to do this for any platform migration.
