I'm not particularly fond of the way React mixes presentation (returning nodes in render()) and logic. Particularly things like:
var classes = _.reduce(["flipped","correct","wrong"],function(m,c){return m+(this.state[c]?c+" ":"");},"",this);
seem very inelegant compared to say, Angular's ngClass, which does the same thing in a data-driven way. But, that's not to say Angular is free from template logic either.
I think the React authors agree that React classes shouldn't contain excessive logic; if your class does, consider extracting it to somewhere more appropriate.
Oh, indeed, I've completely missed the classSet plugin! That would indeed make the code more elegant. And fair point about the logic perhaps being too complicated here.
I find that the ability to trivially make the equivalent of Rails "helper" functions as auxiliary methods on the Component class actually tends to encourage separation of logic - it's the same file so there's no context switch needed. On top of that, since JSX makes ternary statements look much more elegant than bracketed if statements (whose conditions can be a great hiding place for complex logic) it further encourages breaking your logic out into auxiliary functions.
