The "Devil" thing
I thought this was a fairly decent updating and translation into a visual medium of the novel, and I loved a lot of the visuals. But I think they were a little too literal in their interpretation of "the Overlords look like the Devil!" plot point.
The concept is good. Even on earth (or in the same country or city for that matter) what is completely normal to one group may be threatening to another. There's no reason aliens couldn't remind us of things in our mythologies. And remember that even if you believe in demons, the Overlord type of image has no theological basis and has it's origins in late medieval art.
But he looked like he stepped right out of an old European painting or a heavy metal album cover. I would have rather had the Overlords look more alien, but their basic profile (horns, wings, hoofs, tail--or organs that seem to be these things) be suggestive of the popular concept of demons.
It would have also been interesting to know the reaction of people whose cultures don't view evil spirits this way (the vast majority of humans) and the effect or lack thereof on non-religious people. For that matter, the reactions of Satanists and metal fans would have been fun.