One big downside to Google appEngine, is the fact that you can't get your application files back from the server. The development happens at your PC, and you uplad to the server. Now, if you happened to lose your data from your PC, you can't fix your app anymore, even though the files are clearly on the server, and it's serving them, you cant get your files back and keep working on them.
The application is running, but your source is gone. Now, how annoying that could be..

I made my first try at Google App Engine. Basically, I followed the Get Started tutorial, which is something you could do without any programming experience - mostly Cut & Paste.
But then, I added some things, just for practice:
1.Text 'You are not logged in' or 'you are logged in as', near the login link,
2.A favicon, not original, just took one off of a site to see if I can do it.
3. A 'Powered by appengine' button.
4. RSS button, and a feed to go with it.
The last one was probably the most complicated one, though I did something like that before, which helped.

I'm going to check it out some more, maybe something will come out, and maybe I'll write about it here. But then, maybe not.