Apple has posted several samples of Siri's voice in iOS 9, 10 and 11 on its website for machine learning. When you listen to the voices in order of iOS version it becomes very clear how much it has improved. In iOS 11 Siri sound almost like a real person. Even between iOS 9 and iOS 10 there is a big difference.
Scroll down to the heading A New Voice almost at the bottom of the page and listen for yourself: