I use Toucan to learn Japanese and so far, my experience has been great! Of course, like any other language learning app, there are limits to how much of the language you can learn through it, but as far as learning vocabulary (and in the case of Japanese, the alphabets) goes, I think this is a fantastic concept!
Of course, since Toucan is an ever-developing add-on, there are things I would like to see that would make it an even better experience, such as including some kanji along with the hiragana and katakana in the case of Japanese, or translating some phrases to immerse the user even more in aspects of the language such as syntax (but I realize that is probably much harder to implement).
The dev team is really communicative and it seems to me that they really care about providing the best possible user experience, which is another huge plus. I'll definitely let them know about what I feel is missing in the current version.
Full disclosure, the Toucan team generously supplied me with a month of premium membership to fully try the add-on out, but I believe that everything I said also applies to the free version.