Used SRS via Mnemosyne (and later Repetitions.app) heavily in studying for all of my US medical board exams. The effort to payoff ratio seemed very satisfactory.
For pre-clinical rotations, a few nerdy peers and I collaborated on a shared deck of a couple thousand slides -- many of them pathology images -- synced via Dropbox.
For the Step exams, I mostly used practice test questions. Any question I missed prompted me to read up on the topic to determine what piece of knowledge would have helped me come to the right answer, and then figure out how to make a decent card for that principle. Every morning I would start by reviewing all my SRS cards, then do a few hours of practice tests. It was really nice being able to be able to take a core component m of my study material on the road by just bringing my phone! A few of the practice question apps had protections in place to prevent copying text (copy and paste saved a fair bit of time, even if there was also a lot of rewriting) -- but figuring out workarounds like running in a VM or screenshotting from the iOS app was never too hard, and I would just queue up the screenshots to batch process toward the end of the day.
I used a similar technique to pass the tech, general, and extra exams for amateur radio with near perfect scores (the verbatim questions and answers are freely available -- I did try to learn the concepts behind most questions, though a few were admittedly memorization without understanding). Unfortunately my small town is too rural to have a local club, and I have too many hobbies to shell out $1k for a HF rig, so I have yet to make a single QSO. I'll eventually find time to put together the QRP CW kit I got for my birthday :)