One particularly good use case for App Engine is a simple back end for thick client applications that do most of their heavy lifting on the client side.
I'm working on a moderately sized app that uses a fairly complex range of data, but I only have a few tables with a few columns each in the datastore. All the relational complexity of the schema is tucked away in blob fields and handled on the client. Besides greatly streamlining the back end code, it also cuts down on cpu cycles drastically by dumping most work to the user's machine, thereby saving lots of money.
I'm using this approach with a Flex UI, but it applies just as well to javascript.
I'm working on a moderately sized app that uses a fairly complex range of data, but I only have a few tables with a few columns each in the datastore. All the relational complexity of the schema is tucked away in blob fields and handled on the client. Besides greatly streamlining the back end code, it also cuts down on cpu cycles drastically by dumping most work to the user's machine, thereby saving lots of money.
I'm using this approach with a Flex UI, but it applies just as well to javascript.