I’ve been using SuperMemo for 16 years, do my cards every day (just finished today’s cards about 20 mins ago). Super useful for obvious stuff like languages, but also very useful for more subtle stuff you want to keep in your brain; you basically have a “remember forever” button in your life, which you can use for useful information you encounter online, facts about loved ones, jokes, scientific concepts, cognitive biases you are prone to, basically a little bit of everything. Spaced repetition is a very effective tool, but it’s overall usefulness depends on the self discipline and willingness of the user to recognize their own memory related shortcomings. You can live an entire life without spaced repetition and be just fine, but once it clicked in my head how effective spaced repetition was, I couldn’t not be obsessed with it. The feeling of being able to know what you know, and to rely on that knowledge being there, it feels like when Thor reaches out his hand and Mjölnir just flies into it with no effort on Thor’s part. It feels so great!