My Take
It has a variety of ways to learn words and phrases, which makes it engaging for everyone and even addicting for some (me). It has some great built-in rewards and motivational goals that keep you coming back to get to the next level and/or get a higher score than your friends. Students end up actually saying various phrases to each other, laughing about their learning and mistakes, and showing off their newly-acquired knowledge throughout the day, even at recess!
How I Use It
Since a foreign language is not formally taught in my classroom or school, this is one way that students can learn a language. Basically if a student finishes an assignment early, they can go to this website. Or, sometimes I will give all of my students about 20min of dedicated time to do this, and they all enjoy it.