I can state from personal experience that if you are willing to put the effort in and do a lesson or two a day duolingo 100% works. I currently have an 819 day streak, which started with Portuguese before I switched back to Spanish about 6 months in. Before that I had completed the old French, German, Spanish and Italian trees to the first level (of 5) before they expanded the content which meant my trees went to only being a third to two thirds complete to level 1. Yes this was kind of frustrating in that I no longer had a tree complete to level one but it was also amazing: they just added a ton more content to practice. At the end of the day, are you there to complete a video game or to actually learn a language?

Since doing Duolingo, I can now watch Spanish sections of tv with Spanish subtitles on and can follow along with what is happening and being said. I can listen to Spanish sections with English subtitles on and can pick out how the translation is slightly different to what has actually been said. I can think and speak in Spanish and would feel confident going to a Spanish speaking country and interacting with the locals. I have had no formal lessons in Spanish and the only other resources I've used are Netflix (occasionally with the Language Reactor plugin), Amazon reviews, and the odd Spanish articles and tweets. I can also recognise a lot of Portuguese, Italian, French and German words when reading or watching Netflix shows in those languages - nowhere near to the extent as with Spanish but I have not put half as much time in with those languages.

One caveat is that I do pay for premium so I can do all my lessons with unlimited hearts other than the legendary ones which require you to complete them without making more than three mistakes. For me this is perfect: it allows you to learn without stress and then gives you a proper test at the end to prove you've learned it properly. I wouldn't want to do Duolingo without premium: doing the lessons part without unlimited hearts would be very frustrating. For the £78.99 I'm paying for my subscription a year I find it extremely good value for money.

I’ve found the new guidebooks to be very useful and they certainly don’t just contain phrases. The author moans about the fact that the old content and notes are gone: duolingo is doing this explicitly because it is moving to align itself with proper CEFR educational frameworks. You can read more on that here:

https://blog.duolingo.com/how-are-duolingo-courses-evolving/

It is not hard to see that the end game Duolingo is shooting for when all the courses are complete is for a user to make their way through them entirely and then take a test at the end to become certified to B2 level. They already offer the English Proficiency test which is now accepted by lots of universities, I am sure they will expand this for other languages in due time. So far the only ones that are anywhere close to complete for English language speakers are Spanish and French with German a distant third. I suspect this will improve with time. From the looks of things they are perfecting the Spanish/French course and then will update the less popular courses to match. More information on what they are planning to add can be found here:

https://blog.duolingo.com/duolingo-advanced-stories/

The new path is amazing and a massive improvement on the old system. It basically has spaced repetition baked in so you are now learning and memorising optimally. Stories are tied to specific vocab you’re currently learning rather than being a completely random section. Since they switched to the path I have had no issue with my progress disappearing so I don't know if the author just got unlucky or what. Maybe contact customer support? I am currently at unit 94 of the new path out of 211. I suspect it will take me another 1.5 to 2 years to complete fully at the rate I am going (just a couple of lessons a day) and I'm really looking forward to seeing how good my Spanish will be at the end of it. More information on the Path and the decisions behind it can be found here:

https://blog.duolingo.com/new-duolingo-home-screen-design/

To me Duolingo is almost as close to The Matrix vision of downloading information into your brain as you can get. You literally just have to follow the Path and it is downloaded in there. No matter what anyone says though, you can't rush language learning. You are probably looking at 3 to 4 years to truly learn a language fluently. This fits in with how universities approach teaching languages which is typically 2 years of study, 1 year abroad in a native speaking country, and 1 year of further study.

Yes Duolingo had it's problems in the early days and the courses are still incomplete for the majority of languages on there. But if you are an English speaker and want to learn Spanish or French (and possibly German) it is great and I highly recommend it.