Another Korean MMO player here (I play rappelz).
The business model is interesting: basically there is nothing ingame which forces you to buy anything, but buy-only items help in leveling or make you more powerful for a short time, or provide decoration to personalize your character.
From what I've seen it works well. There's a large part of population which is ready to spend a lot of money (I've heard people hitting the cap, which is something like $500/month) either for powerleveling or just to have the cool gear. Another approach is to use this as a money-making approach: want to buy some nice item? Go buy stuff with real money and then sell/trade it to get the nice item you want. This happens a lot, and makes game maker hate RMT ("gold farmers") because they cut into their profits.
What is not clear is the long-term effect. I mean: as the server population increases in level, they have access to a lot of in-game money, allowing them to buy basically anything, and lowering their need to spend real money. Unless there's a fresh arrival of new players who will spend money for the buy-only items the profits will go down. Also, there's a "saturation effect": the decorative items are often buy-once-use-forever: when everyone has one then it's over, forcing the company to keep putting up other (usually more powerful) items.