I don't want to get into a my framework is better than your framework fight, but I disagree with people who say flask can only be used for simple applications. Just because it's a micro framework doesn't mean it's only for micro apps. The trade off on framework size is how much functionality you want out of the box. If you want a bunch of features and are okay with doing it the Django way, Django is great. If you want more flexibility, smaller frameworks like Flask or Pyramid work fine.