You can use Jinja2 with Django, but you lose the ability to use any prebuilt Django templates. Which isn't much of an issue in practice since those are often the first thing you write yourself from reusable apps.

You also can't use templatetags from reusable apps (or Django itself) but it's easy enough to write a wrapper for those that allows you to.

We switched to Jinja2 for Canvas because the Django templates were atrociously slow, and it was absolutely the right decision for us.