Firstly, this service is FREE! I know people who have gone through this process on their own and there were times when they were totally lost. In the end, it worked out for them, but why add that stress when you don't have to?? Moving abroad is already stressful enough on its own, so take advantage of this FREE service.
I had a great experience with Korvia! My goal was to teach through the EPIK program, and I felt more at ease throughout the application knowing that Korvia could anticipate what EPIK would need from me. My recruiter, Jessie, was very helpful! She was always prompt with her responses, which is so important during this process because you want to turn in your application and various documents ASAP. And trust me, there will be many questions. Her encouragement throughout the application process and beyond was exactly what I needed during this stressful time. Now that I am settled in Korea, I know that I can still go to her with any questions that I have.
Another benefit of working with Korvia, is that their various partnerships allow you to receive some great deals! There are some discounts they can offer you that help offset the expenses of applying to teach in Korea. For example getting a prepaid SIM card (for your phone) that will be ready at the airport upon your arrival in Korea. There are other offers I did not take advantage of, but that others have found very useful!
My advice would be to do everything earlier than you think you have to, double check everything yourself, and be proactive! Korvia is an amazing help, but in the end its YOUR application and YOUR experience. They work hard to support you, but you also have to work hard! :)