I've been using App Engine for many projects since 2008. Like any tool, it's not the right tool for every job, but for its intended use cases, it's great. Being able to click Deploy, and not have to deal with sys admin is really convenient.

The one area where App Engine falls a bit short is lack of support for some widely used libraries like Numpy. It would be nice if The Google would add support for those (support for some transcoding libraries would be nice as well). Even better would be an interface to TensorFlow.