It really depends on the project I guess. While you can do everything in the admin, when your app gets more complex using the admin gets more complex too. And if you want to make your app really easy to manage you want control over every little detail, which is not really possible with the admin app.
I guess you should see it like this:
Using django admin: save time writing it, lose time using it.
Rolling your own admin: lose time writing it, save time using it.
