How effective is using Dualingo to learn a new language?

I decided to give myself a crash course in German before visiting my sister, who was living in a little town near the Swiss border for a year. She warned me that only few of the locals spoke English, and that I should try to learn as much German as I could beforehand.
I tried out the Duolingo app, and I had a lot of fun with it. It seemed to still be in the process of working out glitches—the first week or so, I was able to use the verb “essen” (to eat) in sentences like “The bird is eating the mouse,” but this suddenly changed partway through the course, and I had to do some quick searching through the help function to discover that “frissen” was the correct verb in this context. I was also watching the German Language Introductory Course available on Japan’s national broadcaster, NHK, where I discovered that some of the sentence structures on Duolingo might be a bit screwy. For example, some structures in German seemed to have a similar word order to Japanese, but on Duolingo, they followed the same word order as English! However, I found the structure of Duolingo to be highly motivating, since you set your own targets and receive reminders if you fail to meet those targets. They also keep track of your weak points, so that you can reinforce them effectively. Knowing the vocabulary at least helped me to figure things out like signs and German-only websites. I spent maybe 30 minutes or so playing with the app almost every day for close to a month, and my sister (who had been living in Germany for about three months by then) told me that quite a bit of the vocabulary I had learned was new to her.