In these days of holiday I've been playing with the Java version of the Google App Engine (yes I have been lucky to have access to the "early look" version).
In general I find it wonderful. I really like how the Eclipse plugin integrates with the App engine and GWT.
The included services are implemented using standard APIs whenever possible, but you are free to use the underlying technologies in a more low-level way if needed.

My first idea has been to implement a sort of "comet" service for a GWT application, and I must admit: it's not easy because of the many limitations:
1) You can't create threads.
2) You can't use continuations.
3) All threads (generated from http requests) have a maximum lifetime of 30 seconds.

Anyway finally I found a tricky way to implement a sort of slow-polling using the Memcache service, but..
I found a bug: JCache entries' expiration time is not reset when entries are updated! I think this is quite a big issue.

Bug report 1284 already submitted :)