We have a few rather complex mixins in React-Router-Component and react-mixin's decorators worked just fine [1]. The only limitation I see now is a mixin that defines more mixins; this currently doesn't work, but there's no reason why it couldn't. I am considering a patch.
It's just a nice way to split up concerns. In React-Router-Component, the RouterMixin [1] mixes in the Environment Mixin [2] which allows the Environment to address its own concerns in a separate file and to be included in other, custom components.