While I was using Duolingo on my tablet, I was very frustrated with the app because there were no grammatical explanations (so if you're a beginner, you have to learn things via trial-and-error method, which isn't really the best choice) and you had to type in a lot of things, which is a real bother on phones and tablets. I always make a lot of mistakes when I type on my phone simply because the "keys" are too close and sometimes the phone doesn't register every tap... anyway, Duolingo on either a tablet or a phone is a nightmare!
Just recently, I saw my mum using Duolingo on computer and I glanced at the screen. I was very surprised simply because there were more options available! There is even a grammar section! Before every lesson, there is a quick introduction into the basic grammar concepts. My mum had a table with the full conjugation of German verbs which she copied into her notebook. So practical and useful!
Thus, I can only say, Duolingo as an app is a bad choice. It's good only if you are revising the lesson. But if you want to study, you have to do it on PC. When you're on PC, you have explanations - not to mention that typing is made much easier considering that you have a real keyboard to type on. Plus, the screen is wide and you can do other things as well.
What is your experience with Duolingo? Have you ever tried it on PC?