But the fact that AppengineJS (and Rhino/RingoJS that powers it) runs on GAE is a distinctive advantage IMO.
Maybe there's a grain of truth to it, but it doesn't particularly convince me fairly well. First of all, you're presuming that it's a good thing to share code on the client and server side. It turns out that there is such thing as reusing code too much. In particular, I don't like the idea that now not only do I have to worry about browser-specific hacks in my client-side code, but I have to also worry about browser-specific hacks in my server-side code as well.