Route53 provides a vastly scalable DNS infrastructure that's easy to configure, and includes monitoring and management tools far superior to any other provider I've used. The management UI is intuitive and fast, and the hosting infrastructure is robust and reliable.
We were impressed at the number of DNS hostnames available and how well the service handled various DNS attacks over the past few years, much better than our previous providers. We also love the user and account controls, monitoring, and available command line and APIs for managing and working with DNS.
We also have found that managing domain name purchases through AWS has simplified DNS overall for us.