I love PAAS (Platform as a service), it's great to concentrate on the application code and not on servers, configuration, load balancing, auto scale and so on.
I experimented with Google App Engine PAAS a couple of years ago when it was in beta and now that Java and Python are supported and used by millions, I started to use the platform actively (PHP and GO are still in beta).
Here is an introduction SlideShare presentation to GAE with the example code (inspired by Google App Engine team tutorial).
