If you already have some experience with a language, Duolingo lets you complete diagnostic tests to skip ahead within a program. For instance, I took some Spanish in high school, so I was able to test some of the basics and jump ahead a few levels.
One thing I'd love to see is the ability to go back and quickly jump through completed lessons. For now, the app lets you go back and start a completed lesson over, but it doesn't let you quickly skip through questions, which is a bummer if you just want to review something specific.
The types of questions within Duolingo's lessons are consistent, whether you're a beginning or advanced learner. The app asks you to do things like identify pictures, type out dictations, and of course, translate sentences. While the repetition here is certainly good for learning, I do wish the app included a wider variety of question types. At least for me personally, this would have made the lessons more like a game, with things to look forward to. That said, Duolingo does include a handful of game mechanics to push you along. For instance, each lesson starts you off with either three or four stars, and whenever you get an answer wrong, you lose one. Lose them all, and you don't pass. Also, when you complete a lesson, you earn points, which can be weighed against your friends' progress on a Leaderboard.
Overall, I found Duolingo to be a great option for language-learning with a low barrier for entry. It's completely free, and you can easily whip out a few lessons during your sparse pockets of free time throughout the day. There's even an option to receive practice reminders, in case you need extra motivation.
But as someone who has taken language classes in both high school and college, I can't say that Duolingo would be a completely sufficient replacement. It doesn't go in-depth when it comes to sentence structure or verb conjugations. And as with any software-based education program, it doesn't teach you all of the necessary nuances that are required to become truly fluent. That said, I do think Duolingo would serve as a fantastic primer or refresher, and I would highly recommend it to anyone interested in language-learning.