I'm currently using Duolingo to learn Portguese. I've found it to be a very effective way to build up a basic vocabulary and grasp the pronunciation. I plan to start to in-person classes in a couple months once I know ~1000 words. It seems about as effective as anything else to me. In my experience learning a new language is almost entirely about how much effort you put in, rather than the method.
One thing that strikes me from these comments is that many people become focused on completing the exercises and getting points. I can see that could happen, it requires a conscious effort to make sure I understand every sentence and say it out loud.
Starting Duolingo about a year prior to moving to a German speaking country me helped a lot. I can have some very basic conversations at the store and understand ~half the words I read. I think I've managed to not fall in the gamification pitfall by taking the "Learning how to learn" class, which was a good learning catalyst.
I'm starting classes now, and after speaking basic German with the teacher during the intro, she was surprised to learn I only used Duolingo. She said most people didn't progress much with Duolingo.
Someone I know has been doing Duolingo every day for 5y for learning German, but their phrase forming and grammar is still quite bad. I think the missing part is speaking the language and trying to make correct phrases.
One thing that strikes me from these comments is that many people become focused on completing the exercises and getting points. I can see that could happen, it requires a conscious effort to make sure I understand every sentence and say it out loud.