I've thought long and hard about how best to reply to this question. SRS is a bit like marmite, right? Some people love it, some people hate it. As for me, I think marmite's ok but I wouldn't want to eat it on my toast all the time... (wonder how many non-Brits get this cultural reference?)
Anyway, I discovered Anki in the late 00s when I was learning Japanese, which is the first time I really tried to self study a language. For a while it seemed like the answer to all my problems. In particular, I remember when studying A level French at high school we were encouraged to write down unknown vocabulary during the lessons, but having filled up notebooks with all these unknown words, at the time I just didn't get how to turn them into known words. Drilling with an SRS system seemed to solve this problem quite nicely. But then I used it, overused it and abused it and in doing so, I suppose I discovered its limitations.
SRS is just a tool. For some tasks, it can be a useful tool, but it depends on the nature of the task, the nature of yourself as a learner, and the way that you use the tool. Repetition, quizzes and feedback all help with the task of memorisation, and this is where an SRS system is useful. In particular, I find it is useful to make up deficiencies in courses that do not have sufficient repetition, quizzes, and/or feedback, or for aspects of the language which are less "sticky". It's also quite useful if your lifestyle means that you can't sit down for a proper study session every day, but you are able to do so a couple of days a week, as you'll only need a few minutes per day to keep things fresh using SRS in between your study sessions.
Where it becomes less useful, or even detrimental, I think is due to the fact that you can allow it to grow into a monster. The temptation can be do try and put
everything into SRS and before you know it, your study sessions turn into data entry and processing sessions and you spend too much time reviewing your cards and not enough time doing all the other important things you need to do to learn a language. This becomes extra detrimental when you find yourself unable to use the things you've been successfully drilling in SRS, because your brain is used to coming up with the answer when prompted by your digital flashcard, but not when prompted by a relevant situation in real life (yes, this has happened to me).
To take it back to something positive, where SRS has actually been useful to me at the intermediate level:
- Learning how to read kanji words (I honestly don't think I could have done it without SRS)
- Learning noun gender and plural forms and verbs with the correct preposition and/or case in German
For me, successful use of SRS depends on a few parameters:
- Always make my own decks, so the material is relevant to me
- Data entry needs to be as quick and easy as possible - if it takes more than a couple of seconds to make a new card, it's not worth the time
- The time required to review the decks needs to be no more than around 10 minutes a day - more than this and it's not worth the time
- Delete the decks after a few months - if you've not memorised a word after this time, you probably don't need it
- Stick to a daily new card rate of 10 per day if it's one way, or 20 per day if it's two way - if for some reason I need to learn more stuff more quickly, SRS is not the right tool. Good old fashioned word lists are probably more appropriate where speed and volume is more important than long term retention rates.
And remember - SRS is a tool, not a lifestyle!