Before I talk about the show, let's talk about Baek Hyun Jin and his screams/hollers/shrieks... that little dude is part Velociraptor, right? He makes noises I have never heard a human make on screen or off. It's massively entertaining and only sometimes obnoxious. I've seen him do it a few times in other dramas, but Happiness provided him with a great many opportunities to implement his raptor shriek, and it fit very well with his character's cowardly evil personality.

Happiness was ok. It was, for me, just a tad over acceptable but not epic. As other reviewers have pointed out, there are a lot of minor inconsistencies, plot holes, and confusing logic leaps. There are some elements I just cannot wrap my head around the logic of....like why was this apparently government boss dude the literally only guy in charge of the whole outbreak. I can't think of any government that would work that way, there would be lots of officials involved in an operation of this scale. Also why did the relationship develop between the detective and this military boss dude, that part really seemed to stretch the bounds of logic for me. Why too, if the side character brother also had antibodies was his wound purple and diseased looking where the FL's wound was not, where they multiple times in the script bring up the color of the wound's surrounding area as an indicator of infection.

The direction crew did a really terrible job with consistency, so many shots will have the actors holding a prop in their hand from on angle, but when it cuts over to the next angle the prop is gone or in the wrong hand or different. It's pretty jarring and there are multiple examples of this in every single episode. The attention to detail in this Kdrama was pretty bad, and those things break immersion which definitely reduces my impression of a drama.

The acting was pretty good, there were a few sort of one-note bland characters and it was a little disappointing that they didn't delve into any of the backstory on side characters at all, which made it difficult to connect with them emotionally. The ML and FL had good synergy. The FL especially did an awesome job representing her character's personality consistently, so you truly believed her as a character. The action scenes were pretty well done for the most part, though there seemed to be some inconsistency around when a lead could attack a gun holding person vs when they just stood there at gun/knife point. There was a really awesome scene the ML does (or his stunt double probably) where he in one swift move drops to the ground and grapples the preacher zombie's leg, spinning around it to take him down, it was pretty badass.

Overall, it was a pretty good show, though the lack of consistency in the script and directing limited the potential of the actors. Even so, 90% of this drama takes place in hallways, and it's pretty impressive that you can keep people immersed well enough that they remain watching with such dull scenery.