It's really good for its goals. I've used it for a few months and was really happy with the results. The spaced repetition aspect worked perfectly. The courses are still being worked on - already 99% there with quality, but you can report any issues and they get fixed. Just keep in mind that the target is largely students, (at least at the moment) so the aim is mastery of the subject - if you're interested in learning the concept but not actually doing a lot of practice of using it, then it may not be the right service. And there are magic internet points / leaderboards if gamification is something that works for you.

The exercise sizes are also very small almost all the time. That means instead of a whole topic at the time and figuring out where you left the last time, you can do as much as you want at a time and not be restricted by artificial "chapters".