Let me start out with the positives, because there are some. I liked the chemistry between the actors, and it's very easy to tell that they got along on set. I also enjoyed the comedy of this, I found myself laughing a lot during it.
The problem is mainly with one aspect of the show, and unfortunately it's a big one. The writing for this is awful. The choices that the characters are not only nonsensical, they're choices that no human would ever make. The main story doesn't flow, and there's a huge plot shift part way throughout the show. There's also this mystery which I already knew the answer to the first time it's brought up. Another thing which got over very irritating was the music, whenever the characters were silent they would play this music which would describe how they feel. Seriously, we know what's going on, we don't need this music.
I'll talk about the four main characters. The main lead's is one of those clumsy characters you either love or hate, I personally found her irritating, this type of character was done much better in other shows such as Ugly Betty. The male lead...okay I guess, though the actor wasn't as good as his costars. The second female lead had the best subplot, and she has several interesting dilemmas. But the spotlight was stolen by Choi Siwon's character, Shin-Hyuk, mainly because the man was hilarious and the character knew how ridiculous everything going on was. Seriously, Shin-Hyuk needs his own spin off, that's how funny he was!
Overall, this drama does have some good moments, but if you're going to watch it throw logic out of the window.