I'm using it as the backend for my Windows app's sync. So far it's been great. Latency isn't that much of an issue because sync happens in the background, but users report it as very quick.

I'm keen to put a GWT front-end on it. I've tried that on a side project and it worked well. I do worry about the relative unpredictablility compared to just a plain box, but it's balanced with the fact that I really don't want to worry about operating system and web server level security or admin.