It's really really good for what it is, what it is being of course a really good start but not at all a complete way of teaching a language!
As well as approaching it with the right expectations there is also an element of luck - all users are part of a constant trial, and some people get to trial really dumb features
The quality of the courses also varies - some were professionally written, some were done by volunteers of different capabilities (either better or worse than the professional people), some are longer than others, ...
I completed the Finnish course, which is one of the shorter ones. It was done by volunteers and perhaps as a result there are a few minor but from time to time infuriating errors in the English. It did however give me an absolutely lovely solid footing in Finnish, which really gave an excellent grounding that made an A1 Finnish course much more enjoyable.
At one point I had also completed the French course (before it was expanded). It definitely didn't get me ready to go off and have conversations with French people, but it was a useful grounding in being able to understand things.