I used it for 8 months or so, and came to the conclusion that the gamification was completely counter-productive.
One of the big things in raising kids these days is "external rewards" vs "intrinsic motivation". There have been studies that show that kids who already enjoyed drawing, when given a reward for doing drawing, found drawing by itself less enjoyable afterwards.
Duolingo is all about external rewards: And the reward isn't for learning the language, but for completing lessons. I found myself always trying to race through as many lessons as possible; stopping to investigate a word or phrase, which should be rewarded, was actually discouraged by their system.