This isn't deep complex drama, story isn't bad but not so high level. it's one more tale about warriors and friendship.
Star of the show is SML (name of the show is real mistake, because ML and SML are equals as characters/screen time), he and bromance story are main reasons to complete watching drama. Yoo Seung Ho was great and I don't think anyone could not love him. His character is anti hero and really does bad things but is so complex, good played, emotional (though cold as ice) and charismatic (as visually and as a character) viewer develops very strong second male syndrome (though this time it's not problem, I mean drama doesn't treats SML badly as in many dramas, so you can enjoy watching him without being angry on writers).

I started watching for Ji Chang Wook and though I tend to have second male syndrome, this time I really wanted not to have it and like mainly ML, but it's really difficult here. Ji Chang Wooks character is good, hilarious in first part, strong in second part, but he totally goes to shadow next to Yoo Seung Ho (SML). it's really amazing how this guy with most kind eyes and not so masculine body structure can play cold badass mysterious warrior/killer... he plays so believable, even fight scenes - his fighting skills look more perfect then Ji Chang Wooks (K2/Hiller), and till the end you (I know, I was) are sure he would win.

Many characters and story lines are poor and unbelievable. Worst is SFL (or is she FL?) Ji Sun (Shin Hyun Bin), I've never seen so bad acting and so bad character, she is just stone, she is just there, doing nothing, feeling nothing. only time when she kind of was alive/acted(?) and had chemistry was in the beginning and I thought until the end it would develop, but romantic parts (every romance) was most unlogical and unbelievable things in this drama (though it wasn't so important to care).

P.S. Main Lead has that "kindness" and "being right" thing, as many protogonist heroes, when it becomes just being stupid, but overall he isn't bad character and annoying, has his likeble/loveble/cute things too.