1) You will teach alone at times, even though your coteacher is supposed to be there.
2) Your coteacher's level of English may be lower than your students.
3) You should have a basic understanding of Korean if you wish to teach effectively (especially to low level students) and manage a class by yourself.
I'm a pretty positive person, and I wouldn't give up this one year experience for anything since it has tested me in many ways (and I met someone special here). But this experience has been a far cry from what I expected. The biggest issue has been my visiting school, which is a 40 minute bus commute to a rural area outside the city (twice a week). I teach 4 afterschool classes a week without a co-teacher: 1st-2nd grade together and 3rd-6th grade together (the school is really small). If you've ever tried to teach/manage young students (1st-2nd grade) by yourself, you know it's a chore. Add to that the communication barrier and you've basically got a circus going. My 3rd-6th graders are extremely talkative and getting them to listen for longer than 5 seconds is difficult, not to mention finding activities that are level appropriate for all grades. Hence, my first semester was very stressful, to say the least.
Co-teaching is a challenge. Most EPIK teachers' grievances stem from problems with coteachers. You might have coteachers that use corporeal punishment, who can't manage the classroom, who undermine you by speaking solely in Korean, who teach the students incorrect English, or who sit in the break room and enjoy coffee while you teach. Thus, if you actually care about teaching your students English, you might find yourself -somewhat- frustrated. While I've experienced all the above (thus far), I currently have 2 experienced coteachers that I really enjoy teaching with this semester. You get the bad with the good.
With my intake, several issues have come up regarding breaches of contract. In fact, changes have been made to the contract (extending the minimum number of camp days) mid-way through the contract. When I contacted them regarding my summer vacation, they were very unwilling to accommodate my needs on the basis that I must follow the contract. I find it hypocritical that they can adjust the contract boundaries according to their needs but not mine.
As it stands it seems that EPIK offers better pay and vacation days than hagwons do. However, you should be prepared for ANY type of teaching situation, in ANY part of S.Korea. Only apply if you are prepared to be in a less-than-ideal setting.