Any one tried MongoDB on Google App Engine?

You can't run MongoDB on GAE. You need to use the Google datastore.

Matt is right. If you want to deploy to Google's servers you can't use Mongo. If you are interesting in running your own server(s), check out AppScale.

If you really want to use MongoDB anyway, you'll have to find a third-party host that provides a REST interface through which you can make your transactions. I've personally tried and recommend MongoLab on App Engine. Then, in your code, use an HttpUrlConnection object to send/receive requests if you're doing it with Java.