The biggest plus about Google Apps Script is it runs on .google.com, which means you can do things like set calendar appointments, programmatically access Gmail

The biggest plus about Google Apps Script is it runs on .google.com, which means you can do things like set calendar appointments, programmatically access Gmail etc, with the user's authentication cookies. The biggest minus about Google Apps Script is it runs on .google.com, which means they can't do anything that would let you steal people's authentication cookies. This means all your HTML and user-facing JavaScript needs to be run through a sanitizer (Google Caja), which means the performance really suffers, and there are a lot of things you can't actually do. It's possible to build a full web application on Apps Script - someone has even ported Bootstrap to it, and most things work - but most people who try quickly hit the limits and move to Google App Engine.