I've been using Duolingo for years now. I don't think it's effective by itself, but I think it's okay as a supplement to study. Real study or practice with natives always beats Duolingo though. Recently I've tried using a competitor app called Airlearn. Actually it feels like a Duolingo knock-off... made with AI. But Duolingo also have announced they are moving to AI to make their courses. Airlearn has explanations and tips spread throughout, which I really appreciate. Duolingo used to have grammar explanations, but they got rid off them which was a horrible decision. If you use either of these apps, let's add each other as friends!