I think Dart will start to become interesting when they enable the Dart VM in Chrome by default. Then there'll be a real benefit for app developers (a speed upgrade), with a backward compatibility built in (compilation to JS for other browsers). The other benefits (code organisation, language features, etc) are available in other "compile to JS" languages like CoffeeScript, TypeScript, ClojureScript, etc. In that regard, CoffeeScript benefits from a much wider adoption, making it a safer bet. So personally I'm keeping an eye on it, waiting for its integration in Chrome.