It doesn't suck per say... it comes down to the course and your own interest in learning a language. To me Duolingo worked well as an entry barrier for getting started, the app interface is polished and the gamification aspects makes it enjoyable and sometimes pushes you do to a bit more than you'd otherwise have done.
I've been doing the Japanese course for 2 years continuously, during the first year I only used Duolingo but as I progressed and became better at understanding the basics, I sought other apps for the things that Duolingo lacked. nowadays I use 'renshuu' as well for getting a deeper understanding of some areas, and 'kanjigarden' to learn all the various kanji that one can bump into.
Last but not least, I also use Takoboto as a dictionary to regularly look up stuff that's not well explained.
I've been doing the Japanese course for 2 years continuously, during the first year I only used Duolingo but as I progressed and became better at understanding the basics, I sought other apps for the things that Duolingo lacked. nowadays I use 'renshuu' as well for getting a deeper understanding of some areas, and 'kanjigarden' to learn all the various kanji that one can bump into.
Last but not least, I also use Takoboto as a dictionary to regularly look up stuff that's not well explained.