I've got to say that this was one of the shows I was looking forward to and it somehow managed to slip under my radar. Nevertheless, now that I've watched it...I don't have all that many criticisms. The sound quality was decent, the SFX were surprisingly good and the animation looks fluid, except for a few places where the characters are supposed to move quickly (Red Helmet guy getting off his seat and that dude with the assault rifle running towards his friend). There's only one nitpick that I have here and it's the fact that when characters tilt their heads, it looks like the masks sort of slide on their faces, while the skulls stay in one place. I'm not sure what's causing this effect but it may be the fact that you left a black outline around the head.
The reason why this recieves 4.5 stars is the story. You're obviously just setting it up but a 5-star-story gets me from the get-go and this one just couldn't do it.