I've been doing "Dreaming Spanish", which is a comprehensible input service, and using Duolingo as a self-test of sorts. Watching a lot of curated spanish-language content is very engaging, and I believe this method will work (for some definition of work), but it is nice to have duolingo as a fancy flash card system. I think duolingo by itself probably isn't very effective, but it serves as a motivator and i think it's useful as part of a more complete language learning strategy.