I've used Swift to make a basic keyboard framework[1] and I've found it to be extremely crashy and unstable, even in the latest versions. Also, certain Xcode features don't work in it (such as refactoring). Incredibly frustrating. With that said, I love the language, so it's almost worth the pain.

[1]: https://github.com/archagon/tasty-imitation-keyboard

This probably falls on the rant side, but it does show that companies are using Swift:

http://appleinsider.com/articles/15/02/07/apples-new-swift-p...