I took Japanese in college (10 years ago), but had to let it drop after I graduated. At that point, I knew all the kana, some kanji, and could hold basic conversations; my skills went downhill considerably after that. Since then, I've been looking for something to refresh what I knew and put me on the path to learning more: after a few false starts, this turned out to be that app. The lessons proceed at a quick pace, but with plenty of opportunity to stop and practice, or to go back and refresh topics or vocabulary. The kana matching game is surprisingly helpful for keeping in practice. The cultural notes and explanations are top-notch. I've moved on to the next app (and am really hoping to eventually see an Advanced level 3)
I started over pretty much from the beginning, despite many chapters of things that I still could handle from my decade-old lessons. It felt a little slow, but the culture notes were nice, and I ran across the occasional vocabulary I didn't remember, and after a dozen or so chapters I was feeling challenged.
What this app won't do is teach you to speak the language out loud with any kind of real proficiency. Yeah, you can and should practice your pronunciations as you go, but without the ability to have it listen and correct you (which is beyond every other app I've seen, too) you'll need to seek out real Japanese speakers to practice. That's not a bad thing at all, in my opinion.