I've been playing around with DuoLingo for an upcoming trip to Italy. Some thoughts...
First, it seems very well optimized for fun, but not necessarily for learning to actually speak/use the language.
Although it uses spaced repetition and generating semi-random sentences (good!) it does this mostly through multiple-choice pairings of words or matching (bad)
There *does* seem to be tests where one can actually be tested on pronunciation and production of sentences in the target language, so the functionality exists, but I suspect DuoLingo buries it because people find it too difficult or annoying.
Unfortunately, that's *exactly* what you would want if you really wanted to be able to speak it. Real life doesn't come with a list of plausible options to choose from when you're making up sentences. You need to recall everything (pronunciation, words, etc.) from memory.
Thus, I suspect actual transfer from their much-easier multiple-choice-style questions to real situations to be fairly low. It may aid in recognition of sentences, but even then, the restricted domain of vocab doesn't mean much for real listening/reading situations.
The problem is, the matching-and-choosing style of Qs gives you the impression you've learned a lot of the language. Therefore, I suspect a lot of new language learners who aren't using it actively are going to be wildly misled about how much they've learned.
My verdict: Fun and easy to use. But don't expect it to help too much (unless you're digging into a lot more of the exercises that prompt you to write, rather than choose from a list of options).
Pimsleur is probably better, despite using older tech, because the actual repetition of key phrasal patterns from recall is front-and-center, not pushed to the back. Regular flashcards might also work better, since they avoid the excessive hints of the MC-style questions.
As for why DL made these choices which handicap it (despite being incredibly well-designed otherwise), I suspect there's a few reasons.
First, people choose for fun, not efficiency. Hard questions or ones that require typing/speaking aren't as fun, even if they're what you need to do.
Second, computerized grading systems are unfortunately rigid in a way real human beings are not. So if you rely too much on computer grading, the machine might fail you for errors which aren't too important in the early stage.
This is why I think self-graded systems can be good (Pimsleur, Flashcards, etc.) because you can see whether you got the answer right or not. A computer risks being either too strict (failing for punctuation or slight rephrasing) or too loose (incorrect conjugations, preps, etc.)
