It allows you to start with a free plan and then to scale your pricing as the number of users and CPU resources increases. The advantage: you don’t have to setup (and pay) for a huge system to cover the maximum possible work load, but instead just pay the exact work load you need.
So far one had to create the connection between client-side applications, whether from Android or GWT by hand (I know what I’m talking about). This means also to handle authentication issues and so on.
The video below – presented at Google IO 2011 – demonstrates the creation of a task management app with an AppEngine back-end (using the Google Datastore as persistence medium) and an Android as well as a GWT app as the client-side front-end. It nicely demonstrates a new template which an updated version of the Google Plugin creates with build-in authentication and Android to AppEngine RPC call mechanism.