I am trying to choose an auth framework for GAE and would be very interested in other people's experiences on this subject.
I had a look into Shiro and Spring-Security so far. What could you recommend?
Luke Taylor wrote a good blog post explaining how to use Spring Security and GAE together. I believe that both Shiro and Spring Security would be suitable for GAE so it's really a question of what you need. Shiro is simple to implement but limited. In contrast Spring Security is a lot more powerful but this brings some additional complexity. Having said that, recent releases have been much simpler to use (especially since the introduction of namespaces).
Personally I would go for Spring