Like most Korean dramas, this was interesting all the way through. Also like many Korean dramas, it had an overly-large cast of characters and stretched on like taffy. And as some have mentioned, the ending was rather sub-par. That fortunately didn't detract from the rest of the show, but would have preferred a more ingenious twist to the ending, or at least satisfying resolution.
Everyone did a good job of acting, but with such a huge cast some of the characters were somewhat superfluous. One gets the feeling they could have done this on half the cast and still accomplished the same goals. The only reason I mention this is because with the American-unfamiliar names (which aren't changed for purpose of translations), one tends to forget who is who and what role they play. If their role in the series becomes inconsequential, perhaps they didn't need to be in it in the first place.
Still, some people enjoy a large cast of characters, so it's largely a matter of personal taste. I give it 7 stars because of the somewhat weak and predictable / cliche ending... and the typical Korean habit of putting an 8 episode story into 16 episodes. But also because it does maintain interest all the way through.
Then there was the "Superman" bit, with one character suddenly exhibiting super-strength and not really explained. That somewhat jumped the shark.
Calling it a "masterpiece" is a bit of a stretch. It's just basically a typical Korean series. I've seen better, and I've seen far worse. It's worth watching.
