JapaneseAs well as the major advantage of having 5 possible cards, I think where FCD wins over similar programs, is that Ernie truly understands the balance between functionality and simplicity. I think he handles requests for functionality in a balanced way whereas similar systems have just tried to cram everything in. So we have a very functional, intuitive, smooth, solid program; and I am an evangelist for FCD when I talk to other Japanese Learners.
As far as Japanese Study goes, Anki, somewhat stole a march on FCD at the start of the SRS boom, so there are historically more ready made decks available to Anki than to FCD. However Anki has always been glitchy and I find FCD is a much superior program in all respects.
If FCD doesn't have as wide a user base as it deserves, for Japanese at least, I think that is not to do with the program, but with external influences such as ready-made decks, complimentary desktop programs etc.
Flashcardexchange, is the largest repository, but the Decks are often of poor quality, and there are limitations dependent on cost etc. as there are on Quizlet too.
What I think would grow the program more would be a dedicated Flashcard Repository for FCD that can handle 5 sides, without the limitations of the other systems.
Or a growth of the community by collaborative groups through shared Google Docs, with Images/Sounds hosted on Mediafire etc.
I am going off topic now - but to make amends - I will share my own 'FCD'd version of the massive Japanese Core Sentences.
http://www.mediafire.com/?mzqyv5zxcsxvnna,bac3s32fqtfurrd(3318 cards 4 Sides + Audio)
if you want the sounds too, I suggest you store them in a Dropbox
http://db.tt/Br7U3LQ (it's free) as Ernie recommends .
To use the sounds download the zipped folder at the above Media fire address, then delete the Underscore so that there is a space before the word media, and put it in your dropbox folder along with the text file.
Sounds are about 185mb, so if you don't want all those sounds, you can just download the text file and delete the column that references the mp3 files.
A final note, as long as I have been using FCD, I have lurked on the forums and been impressed with how swiftly and completely Ernie responds to questions.