I'm not really the expert on this, since I've only used one, years and years ago before I came to Korea the first time. They got me a dud job that ended with me going to the labor board to get the money owed to me. But that's a long story for another day. In 2 further jobs, I used contacts to get better stuff than a recruiter could find me.
But, as Chris in South Korea says, recruiters are a necessary evil, and for your first job in Korea, it's realistic to expect that you'll use one. And actually, you should read his read his entire post for some good advice.
I'd add ESL Planet to his list of ones that are less sketchy than the others.