Well, Instagram [1] and Dropbox [2] are probably the biggest Python projects in the whole world, and they both are using mypy and seem to be successful.

And my particular experience with it is very positive, even if my project that is using it is still small (it is getting bigger though, and mypy is really helping me to refactor it when I needed).

Most of the "false positives" that mypy reports to me end up being code that is overly cleverly, and rewriting it to make mypy happy also improves the code quality.

[1]: https://developers.facebook.com/videos/f8-2018/type-checked-...

[2]: https://blogs.dropbox.com/tech/2019/09/our-journey-to-type-c...