It is effective, but not without problems.

http://code.google.com/p/googleappengine/issues/detail?id=78... http://code.google.com/p/googleappengine/issues/detail?id=57... http://code.google.com/p/googleappengine/issues/detail?id=78...

Needless to say, it is VASTLY better than Heroku's router. You'll never see an H12 error with GAE.