Basically, you can think of Mnemosyne as a kind of flashcard system. The difference between it and traditional flashcards is that when you review a flashcard, you give it a grade from 0-5. Depending on how you grade the card, the program decides when it will show you the card again.
In other words, you review your cards once when you want to learn them, and after that the program decides when you should review. As you give the program more grades, it gets a better idea of when to schedule the cards.
Using SuperMemo 2006-- software much like Mnemosyne-- I managed to pass the 2kyuu after four months of study. (I had, however, studied Japanese on and off for three years prior to that point.)
But, Mnemosyne has several advantages over SuperMemo 2006:
- Open source-- it's free to use and free to update.
- Great interface-- "it just works"; you can figure out how to use it after only a few minutes
- Fewer bugs-- the program is much simpler than SuperMemo, and has far fewer options, but it is also remarkably more stable.
- Runs on Linux, Windows, and Mac OS X
Mnemosyne will only help you speak Japanese indirectly, but it can improve your reading and comprehension ability dramatically if you use it every day.