I am very interested too ^^
I tried by LiveMocha long time ago, but it wasn't good, there were many errors that even a rookie (at that time) like noticed, so I dropped it... The I downloaded several ebooks and started reading, they were helpful, I'll try talktomeinkorean...
Though I must admit the fastest way I learn a language is by listening to it constantly which I do on daily basis,lol...
As I am really studying Japanese, it leaves me a little space to seriously so Korean, though I must admit Korean has become so dominant in the movies I watch, I sometimes blabber Korean words while being asked something during my Japanese class... ^^ Now I understand more Korean in dramas than Japanese (if my sensei knew this, she's hang me :p)
Thanks to dramas, I understand now 80% of what they say in the drama and can watch them RAW... yay~ though, of course, I'm fluent in Japanese and have a much bigger vocabulary...
As far as I can judge the most difficult part in Korean language is the grammar, as they have ㅓ(eo) and ㅗ(o), and mostly in the words they are read the same way, it can be confusing, also their letter-block building in the words, though through typing it creates the block automatically, I'm confused when I write...
I be more serious in a month, I have to graduate from my university... :p