It depends what you mean by learning Japanese. If it means gaining some conversational ability, then I believe most people would rate it well. If it means becoming profecient or near native level, then I doubt Duolingo could help you do that on it's own.

Maybe 1.5?

It's not that it's completely useless, but it's not designed as a learning tool. It's a game designed to keep you engaged. While it's possible to pick up some things from Duolingo, what it actually rewards with streaks and leaderboard positions and so on isn't learning, but just engaging with the app. It's pretty easy to fall into a trap where it keeps telling you that you're doing well, but you're not actually making progress, because demonstrating progress is completely irrelevant for getting rewarded. And on top of that it teaches some things that are just outright wrong, it even gets kanji readings wrong.

This isn't an easy answer, since it really depends on how you learn. On one hand, it's free and useful for learning bite sized lessons on the go. It also doesn't use romaji and tries to get you on kana very early on. On the other... well... There are a lot of grammatical issues you'll encounter or bad habbits that you'll learn. When I used it, I was having to check the chat logs on pretty much every question to make sure it was accurate. There's also context that it misses out on, like when to use 「は」vs 「が」and other subtle differences. You will not learn the reason for any gramma. You will not effectively learn keigo, figures of speech, or really any practical Japanese skills. All in all, there are some benefits from using Duo, but it should never ever be your only tool. If you're a total beginner, I'd recommend pimsleur or a similar audio program. Once you've learned some basics, you can move on to traditional gramma books, alongside media emmersion (reading, writing, and listening to things in Japanese are indispensable). And with all of this in mind, I think I'd have to give Duo a 3/10. I hope that helps and sorry that this turned into a novel!

It's just not good as a learning tool. I'd recommend learning elsewhere and using Duolingo for reviewing or getting familiar with the language. (and only if you like it)

I (re-)started learning Japanese again after a 20 year break, from when I had it in school. I tried Duolingo for a month, but I grew frustrated with the new design and how it assumed you should know stuff - which was explained afterwards. I ordered Genki (a textbook) and started learning with Wanikani, and that really worked for me. I like the SRS system, and am trying to get into Japanese books by now. (edit: doing WK for ~6months, 2x daily)

This is what worked for me - it doesn't necessarily will work for you. Just shop around and see what you like (eg: paid instructor, online learning, textbooks) and can stick to. Then it's only a matter of time until you pick up the language.