Django's test client is very limited when it comes to testing complex interactions e.g. forms with hidden or persisted values etc. Twill excels in this area, and I am gald to have discovered it is now very easy to integrate it without having to abandon any existing test suite or Django's method of running tests and doing fixtures etc. Have a look at my snippets for calling Twill from within Django unit tests.
Disclaimer: most of the hard work was done by 'Mick' in Basic Twill intercept testing of Django and the Twill folks for making this possible.
Update 2017 - Twill is no longer updated. Checkout django-functest if you are looking for this kind of testing.