On the parody side of things, Extermination is pretty funny. Promethean's good for one joke and Nox's flashback this issue is ripe with Batman allegory and even touches on just what would happen when someone with superstrength has sex. This issue isn't guffaw worthy but did lead to a few chuckles.
On the moral side of things, we've got two arch nemeses, Nox and the Red Reaper, forced to work together. Their ideas of what's acceptable to survive are very different, but it should be noted that neither of them is a saint in this world. Each of them will even go out of their way to antagonize the other. Red Reaper's also interesting in that I don't have a clear cut metaphor for who he represents. He's almost simply villain incarnate. The character examination between these two is interesting and worth watching to see how their opposite philosophies change given the harshness of the world and long-term exposure to one another.
I found the art simplistic but adequate to getting the point of the story across. The characters are frequently seen in unnatural poses, but I wonder if this isn't meant to be a parody of the superhero genre in itself (and for a change of pace these characters are all male, not female). It's also worth noting that the pdf I had of the book had a low resolution, and that the art looked better in Nox's flashback scenes. Having missed the first two issues, I will say that Extermination #3 gives enough background to get the gist of what's going on but is not a great jumping on point, so I recommend starting back at Issue #1 if Extermination is something you'd like to check out.