Personally I see the value of both. Depends on the codebase.

classes-or-objects.js

// mixins with createClass:
const Foo = createClass({
  ...someMixin,

  render: ({ onClick, children }) => (
    <div class="foo">
      <button onClick={onClick}>Click Me</button>
      { children }
    </div>
  )
});

// mixins with class sugar + decorator (see below):
@mixes(someMixin)
class Foo extends Component {
  render({ onClick, children }) {
    return (
      <div class="foo">
        <button onClick={onClick}>Click Me</button>
        { children }
      </div>
    );
  }
}


// Here's a simple decorator that satisfies the above usecase:
function mixes(...behaviors) {
  return target => behaviors.forEach( b => Object.assign(target.prototype, b));
}