Django is used on some very large and high traffic websites and is just as capable (if not more so) of scaling to large applications. Arguably, you could say that learning Django is a waste as it may not provide you with much advantage if you already know Rails - however, if you are looking to move away from web development perhaps, there are more jobs in Python and learning another language is always a good thing anyway.

The Django documentation is fantastic and I recommend you give the introduction a go: https://docs.djangoproject.com/en/1.4/intro/overview/