http://www.exratione.com/2010/12/how-to-block-999-of-all-mov...
It works very well unless you're big enough to merit individual attention from a spammer. It's not rocket science - it just raises the bar a little above the level of effort that people who spam everything, everywhere are willing to put in.
That might change.
The real merit of Javascript used this way is that there are so many different possible approaches and ways to write the code that parsing has to be done on a site-by-site basis. It should even be possible to write something that auto-generates -and-mixes various combinations to make it annoying and costly for an individual to keep working at breaking the protection, and thus increasing the size of community/site you could protect this way.