These are all neat, but they require some effort to bootstrap. The neat thing about Brython is that you can start writing Python immediately. I’ve played with ClojureScript and it takes a while to get the QuickStart to work.

I think Brython is cool, not from an efficiency or machine-performance point of view, but from a human point of view. It makes it easy to start hacking. The barrier to entry is non-existent.