I learned Python on App Engine. After a couple of projects I got paranoid about "lock in" so I rented some server space and learned vanilla Django and did a few projects. Only then did I learn to appreciate App Engine.
Fast forward a couple of years, whenever I need a server for a mobile/desktop app I'm building I use App Engine. For many things, especially if you're just starting out, you can't beat App Engine's ease of use and cost.