Admittedly I haven't tried Duolingo but I keep hearing about it from friends (like @jtzou).
I have paid several hundred dollars for Rosetta Stone. Duolino is similar in method and just as good. I have used both for basic Spanish and IMHO they are equivalent.
Rosetta Stone has more languages and may be better at more advanced levels.
I liked Duolingo as a refresher for some basics. Helped me get things back. I'm not sure about progressing with it, as I haven't tried anything new, but the form factor is nice, it's fun to do, and if you're consistent with it, it can be effective.
I purchased Rosetta Stone a bunch of years ago for several hundred dollars. It wasn't nearly as helpful as Duolingo. The #1 feature of http://duolingo.com is having it on your phone. Got a few minutes waiting for a friend? Learn some Spanish! ❤️ it!
Just tried it out for Spanish and loved how easy it is to use. They help train you on reading, listening and writing and the lessons are fun and engaging. Definitely will keep going for the next few weeks to see how it compares to Rosetta Stone
Duolingo is revolutionary. My wife uses to learn english and it's fun, we both interact and use. Great tool.
Late to the party, but I've been using Duolingo for a week now. My 2 semesters of dormant college Spanish have come right back. I love the format and the ease with which you can do lessons no matter where you are.
I've been using Duolingo for years, to practice and increase my fluency in Spanish. Much easier to use on a computer than phone however.