The characters in this show are amazingly written. They all have backstories that support how they were shaped up, they each have something that defines them, some of them even have a shtick that has become a running gag and all those characteristics are used very well during the show. More than anything they are consistent characters. Whoever wrote them did a great job!
The story itself was not something extraordinary but the characters do so well throughout the show that the story kinda takes a secondary place, at least for me personally. Apart from the writing though, the actors did their very best. Jin Goo was phenomenal at playing such an eccentric role. He was mysterious some of the time and grounded at other times, but mostly eccentric and uncontainable. Seo Eun Soo was very believable(even though her role was a bit annoying at times but that only goes to show how well she performed). Yoon Park and Chae Jung Ahn had their own level of uniqueness(and Chae's role was a badass). Jung Sang Hoon started annoying for me but the more the show progressed, the more I liked him. At the end he was just as phenomenal as Jin Goo for me personally. Lee Soon Jae was brilliant as well and and matched very well with Jin Goo's character as he was like a mentor to him and the only person that could really set him straight. Lots of comedy in this show by many of the actors.
I'd rewatch it later on just for the performances alone.