The steadfast will feel like this is abject capitulation, a bitter retreat from the principles that bind you to your favorite syntax. Some may even feel it's a bit of a betrayal, an act so traitorous that you must hide it from your colleagues. Others will suggest, quite correctly, that it's not so simple. Getting the code to run is one thing. Gluing the parts together and creating a UI is plenty of additional work.
Whatever the reason, they did it and they often use it for their most sophisticated Web products. It's a great gift for anyone with a pile of Java and no time to rewrite it. The guts of the language are all there, but some of the less common classes like BigInteger are missing; that said, you can usually find a way to add them. The user interface framework is largely borrowed from Swing, so Swing developers will feel right at home. Others won't find it too hard to learn.
Google Web Toolkit is far from the only choice for Java programmers. Java2Script is fully integrated with Eclipse, and GrooScript converts Java's kissing cousin, Groovy.
Erlang lovers have several options. One solution is to use Erjang, a tool that runs Erlang on the JVM, which is then used to run Java byte code with one of Java options above. It sounds simple, right?
A third option is to use LuvvieScript, a strict subset of Erlang that's been given hooks to access all of the DOM objects. You use the Erlang structure you love, and it translates your instructions into something the DOM understands. It's not exactly the same, but it will do.
The most fun, though, may be PyPy, an incredible feat of software engineering with almost Rube Goldbergian proportions. Python goes in and runs on a Python interpreter written in RPython, a subset of Python designed to be easily compiled. This RPython is then compiled down to something that looks like C that can be fed into Emscripten. The developers claim that they can show some Python benchmarks running faster in SpiderMonkey than CPython.
If they can do that with Python and C, you can too.
The CoffeeScript world is remarkably diverse. Once the world realized it could preprocess its code, many got into the game. Iced CoffeeScript, for instance, is like regular CoffeeScript but with a few extra constructs that make asynchronous calls a bit cleaner and simpler to type and read. There may be at least a dozen cousins offering to simplify your particular style of programming.