I like the ability to customise the lessons (- even fine-grained adjustments are possible too), the omnipresent cute Kao-chan animations & drawings, the grammar & sentence lessons/examples, games & competitions held regularly, and the very helpful fellow-users.
I love how everything can be tailored to your needs and is also SO CUTE at the same time! And I can finally freeze schedules if I need to. KAO CHAN IS SO AMAZING!!! And the people in the community are so nice!
Because it has everything a Japanese learner could need (well, except for kanji writing for me, but I use kanji study for it). It's literally the only app you need to progress from hiragana to N1, it's THE Japanese app. Another big one is that it doesn't overwhelm you unless you ask for it. Also, I really enjoy the different motivators
I have just joined the app, but get the sense that it will help me in learning Japanese, even a little. The presentation is very straightforward and practical, but also comes with cute little side games to motivate you, not taking away from the purpose and only adding to it.
KAO CHAN IS SO AMAZING!!! 
And the people in the community are so nice!
and i also love developers, they did great work, thanks to them!