I've done the Duolingo Spanish tree, and my feeling was the same as the author's regarding "% fluency," which I thought was kind of a joke. Especially for speaking and listening, there's really no substitute for immersion and real-life regular interactions.

But I do feel that Duolingo was honestly pretty good about helping me to read and write. In NYC there's a little ambient Spanish everywhere and I went into it not with the goal of being fluent, but just being able to read signs and understand a little bit here and there. (I haven't kept up with it and now I've forgotten a good chunk, but I'd say that's a different issue.)

At least in my case, it's also been a good entry point because it's free and convenient. I'm actually working on Duolingo's in-beta Czech tree (significant other's family is partially Czech) and it's actually been an inspiration for me to inquire about in-person classes, especially knowing that if I ever want to hold conversations, I will need to have real-life speaking experience. I probably wouldn't have done this if I hadn't started off on Duolingo and had some fun with it!