I've run a few small side projects on GAE (Google App Engine). I use it as a very simple and fast way to get an MVP off. My major issue is that their documentation tends to lag (sometimes way behind) when they change stuff, especially things that are integrated to other services they provide. For example after dropping support for OpenID 2.0, the documentation for 'Authentication Type' on Google App Engine was not updated to reflect this, neither was the documentation for securing your urls. This cost me considerable time and effort.