This started out well, with a stylish opening sequence, a less common take on a monster theme, and a cool FL (Han Hyo Joo, who I almost didn't recognize - I didn't like her much in W, but she's great here), who continues to be cool all the way through! - and doesn't turn into a crying fainting jelly, as so many of them do. It is a fairly silly plot with many holes, but hey, it's a monster show, I can hand-wave that if the pace is good enough to glide me over them. Unfortunately, the storytelling in the middle fell rather flat. There are a lot of side-characters - naturally, as it's the story of the apartment block - but none of them felt compelling: no solid arcs, just randomness and dead ends, and too much time given to the least interesting and most annoying of them. (actually that was almost all of them! and they killed off 15th floor guy, who I kinda liked) The spurious melodrama with these unappealing goons messed with the momentum enough for me to get bored and abandon it on-hold for months before finally finishing it. It does pick up towards the end: the last two eps were better, and the finale tied up some of the essentials, including the romance arc, but that felt unsatisfying to me and I would have liked to have known what happened to other characters.

Still worth watching, but I'd keep the skip button handy.