There are restaurants all over the place, with a huge menu that are cheap, fast and a good place to try new things out. Usually they're called kimbap 김밥 something, nara, mania, pasa, cheonguk... and usually they're orange and look something like this:
And here is a picture of boribap (ok, I eat a lot of bibimbap/boribap) that I got at a restaurant that specializes in boribap and kalguksu:
Kimbap places are everywhere and have a vast selection, making them a great starter place, for those unfamiliar with Korean food, to build up a repertoire of good dishes. But for the best food, you need to go to a place with a downsized menu. And when the prices are comparable, and you're comfortable with Korean food, why wouldn't you?