Sharing models between Django apps

The answer is yes. It's perfectly okay for one application inside your django project to import models from another application. The power of a django project lies in the apps and their interactions.

Also make sure that you have utility applications importing models from more generic applications and not the other way. So "userstatistics" app should import models from the "users" app but "users" app should not rely on the "userstatistics".

If your app is importing models from a 3rd party application (lets say django-piston), be sure to specify that in a requirements file.

If you're building an internal app that has no chance of ever being released to the public, sure, do whatever you want.

If you're building an internal app that has little chance of ever being released to the public, but will possibly be used by future/current developers, sure, but be sure to document what the app needs to work properly.

If you're building an app for public release, try to keep it self-dependent (and django-internals dependent, ie, use what django provides, when possible). If you really need a third-party app to work, or if a third party app would make your code more manageable, then sure, include dependencies, but be doubly sure to document all requirements and necessary setup.

In most cases, you can do almost whatever you want so long as you have sufficient documentation.

I do, however, have to question the sanity of making your own User model that has the same name as django's builtin auth.User.

You cand try better extending the Django User model with inheritance. You will use the django user with custom field added, so you will have the same user for all the apps.