I don't crave hamburgers often here in Korean, because there is too much other food that is just so much better. However, last week I ate a hamburger for lunch on Wednesday and Thursday while getting film processed in Wangsimni.
Lotteria and Kraze Burger are both Korean fast food chains, although their hamburgers are very different. At Lotteria I ate a Steak Hamburger, it was topped with cream of broccoli and on the bottom had sauteed onions, better than anything at McDonald's. But, it didn't compare to the Maximum Burger I ate at Kraze Burger the next day. One hamburger at Kraze costs more than an entire set (meal) at Lotteria, but it is so worth it.
If you can't find a Kraze burger you're better off eating gimbab than at Lotteria (even though their steak burger isn't bad).