[Cyclamen]

It's not really hard. Japanese and korean grammar are quite similar, same phrase structure, very similar verb conjugations, etc.
The writing system is simply brilliant... I learned hangul in just one night, and that's all I'll ever have to know!!
I studied korean with a bunch of people who already knew japanese or chinese. Well, people who spoke chinese had some difficulties, but people who spoke japanese had no problem at all.
I'm not saying you'll master korean in a weekend... as every language, it will require constancy and time spent on studying. But you'll never ask yourself "why they put this word here?" or "why this form?", you'll already know.