Should I use Django's contrib applications or build my own?
It all depends. We had a need for something that was 98% similar to contrib.flatpages. We could have monkeypatched it, but we decided that the code was so straightforward that we would just copy and fork it. It worked out fine.
Doing this with contrib.auth, on the other hand, might be a bad move given its interaction with contrib.admin & contrib.session.
The Django apps come with their own features and design. If your requirements don't match 100% with the features of the contib app, you end up customizing and tweaking the app. I feel this involves more effort than just building your own app to fit your requirements.
What do you think?