As this film is another in a long list of Asian horrors to get fed through the Hollywood tree chipper, some people may well be looking for the original to see if it's any better than the remake (which isn't difficult, as the remake ignores 90% of the film out of hand). However, it isn't a genre classic by any means.
The problem with a lot of Korean horror is that, quite frankly, they're not that interesting, especially compared to some of the masterworks from Japan in the last decade or so. Indeed, they often come across as not very good copies of J Horror title, for example Phone is basically Ring, A Tale of Two Sisters is (essentially) Dark Water, and so on. And this film falls into that category.
It starts off interestingly with the opening mirror murder, which is used as the cover art, but at the same time you have to ask why somebody would buy their friend a pizza wheel for their birthday in order to set it up. Indeed, as well orchestrated as the mirror murders are, they require a lot of contrivance in order to happen in the first place.
It's a waste of an interesting conceit, that being a world on the other side of the mirror, and it does lead to some interesting plot developments in regards to one character's back story, yet it also leads to that other problem of Korean horror cinema: less an ending, more the film finishing.
If you're a completist, this may serve as a good addition to your Asian Horror Collection, but the film is pretty weak in its execution and wastes a conceit that could've been better expanded upon in another director's hands.