At 00:40:28:

If you have not ever used a functional language or ever used persistent data structures in a non-functional language, just take my word for it: This is so much better! If you write a program that uses data structures like this you will just be able to sleep at night. You are gonna be happier. Your life is gonna be better, because there is a huge quantity of things you will no longer have to worry about.

Rich Hickey: Are We There Yet?