MovieGuys
I found it difficult to relate to Alienoid.
I suspect the reason is cultural. Some things work across cultures and other's don't. I found Alienoid simply too strange. It has an off beat personality too, which really didn't help matters. The slapstick humour fell flat, too. Giving characters names like "dogturd", is a bit juvenile.
Action wise it's alright but the action often didn't make a lot of sense, either. Characters appearing out of fans and hiding behind magic veils, just didn't track well, with this viewer.
In summary, probably one for the domestic South Korean market. Lots of elements I suspect reflect Korean mythology but without a meaningful context, its lost on Western audiences.