Just discovered this feature on a website that uses it for its internal search engine, played with it for a little while, and am enchanted by it
Instead of building your search engine with tons of LIKE statements, you can just add a WHERE statement. Example:
SELECT * FROM articles WHERE MATCH (title,body) AGAINST ('PHP content management system');
would select all the articles that deal with it, with the results being sorted by relevance, first result being the most relevant, and so on.

Anyone using that cool built-in functionnality ?