> It produces readable javascript that is reasonably debuggable

The examples on the page don't seem readable and reasonably debuggable to me. The simple square definition:

  square x = x * x

Compiles into:

  var square = function($_a) {
    return new $(function() {
      var x = $_a;
      return _(x) * _(x);
    });
  };

Which, in turn, has all the "$" and "_" indirections.

This is basically why i think that trans-compilation to JS from a semantically very different language is not a good idea, unless a way of debugging _in the source language_ is provided. The success of CoffeeScript, i think, comes from it not differing too much from JS semantically; yes, it adds things like classes or everything-is-an-expression semantics, but the step of trans-compiling those things into JS is pretty trivial.

I hope the advent of source maps will help in the ease of use of these languages.