http://code.google.com/appengine/docs/whatisgoogleappengine.html

Google App Engine is the perfect tool for python developers in dire need to move their infrastructure to a Python based Cloud. It is still new and growing. It provides static and dynamic generation of content to a user with minimal effort. Once it matures into a real product and branches out its available languages, it may be the tool for you.

Tutorials

http://code.google.com/appengine/docs/gettingstarted/

Example Sites

http://www.hypetape.com/
http://www.tweetwheel.com
http://www.blografica.com
http://www.congregar.com/
http://codereview.appspot.com/

A simple SDK with included server allow you to write and test without interacting or uploading to the Google App Engine. Additionally, a configuration file app.yaml allows the application to run specific scripts given the particular URL passed. I was not able to find a large amount of code from actual users. However, the ease of start-up and running code makes this semi-trivial. The restriction of languages to only Python may have thrown off some developers, but it appears as if more languages may be in the works.

The tutorials are simple and straightforward. Example code is provided for some common use cases such as AJAX, Django, e-mail, and authentication with Google. In fact, the integration of some Google services with the application streamlines the process of creating a social website.

Available code is scarce for the lazy. Examples make writing your own application the matter of a single day. Not to mention that almost the entire Python library is available to you. There are some libraries which are restricted due to references to C and calls to the underlying OS.

There are some very interesting ideas and designs displayed at the application gallery.

http://appgallery.appspot.com/

Many of the applications appear for the singular purpose of enjoyment. I had a great deal of fun poking around people's strange toy developments. Most of the services I encountered were far from mature. Although, the Google App Engine is currently free of charge for all users beneath the space and bandwidth quotas. As long as your application does not become too popular, expect a rewarding experience. Page load times appear to be somewhat lengthy on the demonstration sites. However, it may be due to the dynamic content of those pages.

Some other popular reviews of Google App Engine
http://www.techcrunch.com/2008/04/08/techcrunch-labs-our-experience-building-and-launching-app-on-google-app-engine/
http://www.niallkennedy.com/blog/2008/04/google-app-engine.html