Mnemosyne is very, very good software. Using Mnemosyne, I learned to read the 1945 kanji for everyday use. But, making flashcards is not a substitute for traditional studying.
I've seen a lot of conversations on the Internet that suggest if you build the perfect set of flashcards for a program like Mnemosyne, you'll be able to learn any language with ease. This is patently false.
Flashcards do one thing: they help you remember. Before you can remember something, though, you have to know it. You have to understand it. If you try to memorize something without understanding it, you aren't increasing your knowledge. You're just memorizing, and it won't help you speak or understand the language that you're trying to learn.
That's one of the reasons why it's so important for you to make your own flashcards rather than using someone else's set. Only you know the mental links within your own brain. Trying to work with someone else's is inefficient and unnecessarily difficult.
Of course, Mnemosyne is a great program, and I highly recommend using it every day. Just don't think it can replace studying in the traditional way-- taking classes, reading books, and talking to native speakers. It's a supplement-- it allows you to not forget what you've learned, and in that way, it's the best supplement-- but it is only a supplement. For the real learning, you need to experience the language directly.