I tried Busuu's free trial first in learning Portuguese, but it's not very useful on its own as you still need to do your own research or study outside the app. I do like its feature of having my short exercises get corrected by native speakers, although the only interaction you can have with them is through those exercise corrections, nothing more.
I'm trying Mango now too. It's pretty cool too. Its style is different from Busuu. I can't really say one is better than the other but I think they complement each other for me to get a better guide in studying the language. I also watch some Youtube vids.
I think you can't rely on just one app. You have to take down notes as well and organize everything you learn from vocabulary to grammar. And you also need to find a way to actually speak the language, and that is why I am trying italki now. :)