Amazon is one of the largest companies on Earth and they've been getting into AI/Machine Learning recently. I've seen many related products from them. One thing I have faith in Amazon Polly is to sink enough resources to get things right. The use cases are of course massive. Just imagine what this would do to people with speech impairments. This would also make conversations with deaf people people even more pleasant by using Hand Signal > Text > Speech converter.

Multiple language support is also highly appreciated. The best use case for me would be making those product promotion videos and narrators. It's a really hard job that many are not capable of. You have to try it to know how difficult it is. From now on you can type instead of trying.

The pricing is also quite cheap compared to the convenience.