I've used it to add test coverage. Granted it wasn't for new major features but small features that still necessitated having tests.
IME so far that's what it's best at. As long as there exists tests in the codebase that provide enough of the basic skeleton (i.e. if they already test part of a controller or code that interacts with a DB) to guess what's needed then it can do a decent job. It's still not perfect though and when it comes to inventing bespoke tests that are uniquely different from other tests it needs much more of a helping hand.
