I think they are serious examples. Of course once they get to a certain size the cost of running on App Engine becomes exorbitant. At that point, there's two options:
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.
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.