Another point is that the Captcha-image of the free version does not work in mobile-browsers (neither in the standard browser of my galaxy s3 nor in google chrome for android). It only shows an empty field with a question mark in it. If that would work too (or maybe there is a way which i haven't discovered yet..) then i'd totally rate it with 5 stars.
But all in all for the use in desktop browsers: really, really great!
The only thing that bothered me as a German-speaking guy is that words with charachters in it such as ä, ö, ü or é, è, ë do not get linked in the articles.
I would give it 5 stars if the developers would include those kind of characters as well.
Use UTF-8 encoding on site!