Also, How usable is this for building GUIs on Windows? Does it play nice with QT creator?
(I'm still inexperienced with QT development so please bear with me)
This project is different from most language bindings. With it, you generate a binding just for the code specific to your project. So interface the between Rust and C++ is very small. This project does not wrap Qt API in Rust code or Rust API for Qt. Instead of using a lot of energy to heat the whole house, you put on a sweater.
Having official status, means anyone coding for the said platform can be confident that everything related to the platform infrastructure (IDE, libraries, debuggers, documentation, discussion forums, build systems) will take into account the language.
Anything that is just 3rd party will have integration issues, as one needs to rely on the language community to step up to sort them out, or even left in a zombie state as interest fades away.