LessWrong seems to be a big fan of spaced-repetition flashcard programs like Anki, Supermemo, or Mnemosyne. I used to be. After using them religiously for 3 years in medical school, I now categorically advise against using them for large volumes of memorization.

[A caveat before people get upset: I think they appropriate in certain situations, and I have not tried to use them to learn a language, which seems its most popular use. More at the bottom.]

A bit more history: I and 30 other students tried using Mnemosyne (and some used Anki) for multiple tests. At my school, we have a test approximately every 3 weeks, and each test covers about 75 pages of high-density outline-format notes. Many stopped after 5 or so such tests, citing that they simply did not get enough returns from their time. I stuck with it longer and used them more than anyone else, using them for 3 years.

Incidentally, I failed my first year and had to repeat.

By the end of that third year (and studying for my Step 1 boards, a several-month process), I lost faith in spaced-repetition cards as an effective tool for my memorization demands. I later met with a learning-skills specialist, who felt the same way, and had better reasons than my intuition/trial-and-error:

Flashcards are less useful to learning the “big picture”
Specifically, if you are memorizing a large amount of information, there is often a hierarchy, organization, etc that can make leaning the whole thing easier, and you loose the constant visual reminder of the larger context when using flashcards.
Flashcards do not take advantage of spatial, mapping, or visual memory, all of which the human mind is much better optimized for. It is not so well built to memorize pairs between seemingly arbitrary concepts with few to no intuitive links. My preferred methods are, in essence, hacks that use your visual and spatial memory rather than rote.

Here are examples of the typical kind of things I memorize every day and have found flashcards to be surprisingly worthless for:

The definition of Sjögren's syndrome
The contraindications of Metronidazole
The significance of a rise in serum αFP

Here is what I now use in place of flashcards:

Ven diagrams/etc, to compare and contrast similar lists. (This is more specific to medical school, when you learn subtly different diseases.)
Mnemonic pictures. I have used this myself for years to great effect, and later learned it was taught by my study-skills expert, though I'm surprised I haven't found them formally named and taught anywhere else. The basic concept is to make a large picture, where each detail on the picture corresponds to a detail you want to memorize.
Memory palaces. I recently learned how to properly use these, and I'm a true believer. When I only had the general idea to “pair things you want to memorize with places in your room” I found it worthless, but after I was taught a lot of do's and don'ts, they're now my favorite way to memorize any list of 5+ items. If there's enough demand on LW I can write up a summary.

Spaced repetition is still good for knowledge you need to retrieve immediately, when a 2-second delay would make it useless. I would still consider spaced-repetition to memorize some of the more rarely-used notes on the treble and bass clef, if I ever decide to learn to sight-read music properly. I make no comment on it's usefulness to learn a foreign language, as I haven't tried it, but if I were to pick one up I personally would start with a rosetta-stone-esque program.

Your mileage may vary, but after seeing so many people try and reject them, I figured it was enough data to share. Mnemonic pictures and memory palaces are slightly time consuming when you're learning them. However, if someone has the motivation and discipline to make a stack of flashcards and study them every day indefinitely, then I believe learning and using those skills is a far better use of time.