It was good until the episode 9, in the first part, the aliens started to attack the cities, I was asking myself how they could survive or save the other people. In the second part, we saw the had grown up, they were working together and there was a chance they could eliminate most of the creatures. The only problem was in the episode 10, it started to be nonsense, when Gook Yeong Soo started to be strange, just because he wanted to go to the university of Seoul, and from nowhere, he killed one of the students (I don't remember his name :) ) it was okay at first but when he killed the other ones, I felt angry, I was like why did the production do this to the characters, they played well their role and they decided to make them all die, I really don't understand their choice. One other thing that is a disappointment is the lack of logic in the plot, in the second part, we don't know where is Kim Won Bin, he disapeared and we don't see him again, after the prison scene, we don't see a single alien, if we make a conclusion, Gook Yeong Soo killed more people of the group in a few hours than all the aliens in the whole serie, Gook Yeong Soo did in one episode what the aliens tried and didn't acheve to do in the whole serie. Also after almost everyone was killed in the group, two month later, we learn that a weapon was invented, that almost all the creatures are dead and that everything is back to normal, just like if everyone was killed for nothing. One last thing is the music, in the whole serie there is the same music and the put it at the end also, in the last episode, I think it souldn't be in the last episode.
In conclusion, you can watch the serie if you want but you will be disappointed by the last episode even thought it is a good serie in global.