I’m definitely pro-Duolingo biased. However, it’s helped me tremendously. I did a lot of independent study but couldn’t string sentences together before. I have a much better grasp of sentence structure, now. And that’s where their strength lies.
They’ve recently added the lesson notes to mobile (it should be there for everyone, now, I think), so that’s extremely useful. It’s not a textbook, but it’s a general overview. They used to only be on the website. The sentence discussions are invaluable, because smart people like Wantitled explain things very well.
The main weakness, IMHO, is the lack of kanji. Test out of the levels you don’t need
The curse as it is now is designed to get you to JLPT5, which also has very limited kanji. However, they are currently working on version 2 of the tree, with more advanced levels and a ton more kanji. I have no idea how long it will take, but they were looking for contributors a couple of months ago, and their desired qualifications were ridiculously high.
I also have found the Korean course to be too difficult, so I’m coming back to it after I study more. But I personally love the Japanese course, and I love Duolingo. I’m quite involved with them (I’m a Global Ambassador trifecta - event host, forum mod, and teacher).
A point of clarification. Duolingo is not “freemium”. You can access all of the content for free. That’s part of their mission, and they’ve turned down load of money from investors who wanted to change that. Duolingo Plus is ONLY to remove ads and allow you to download lessons to work offline (it comes with a couple of other perks, like a free monthly streak freeze). But all of the core content is, and always will be, free.