I'll readily admit, I had to play through Dead Space before I was able to truly appreciate what Downfall was going for, but even without the link to the game, Downfall was still able to stand on its own. Aftermath isn't as strong.
Aftermath is meant as a bridge between Dead Space and Dead Space 2. The planet Aegis-7 has been reduced to little more than a rock collection and the only four survivors have taken refuge in the USG O'Bannon. The group is "rescued" and forced to relive their plight though a series of brutal interrogations. Unlike Downfall, which barreled straight ahead with its story, Aftermath weaves its story around each interrogation, slowly filling in the gaps in the USG O'Bannon's last voyage.
I get what Aftermath is attempting to accomplish through its plot, but it never goes anywhere. A majority of the movie is made up of people talking about a shard of an alien artifact that causes people to go insane. The premise is interesting enough, but Aftermath insists on jumping from one cliché to another without any payoff in the end. The sort of plot structure Aftermath is chasing only works if you care about the characters. Aftermath tosses in four bland characters and spends nearly an hour and a half shuffling them around without any sort of satisfying resolution.
Considering Aftermath's gore-filled pedigree, I at least expected to see a few blood-soaked action sequences. Necromorphs only show up during the last twenty minutes of the movie, but by then it's too late for any of it to matter. There's no tension and it comes off as an afterthought. Aftermath does, however, offer viewers a lot of "f-bombs" and any other four-letter word it can cram into each line. Foul language is cool and all, but only when it actually serves a purpose. It's overused to the point of distraction and only serves to deface an already weak script.
Even more distracting than the language is the two animation styles used throughout to distinguish the survivor's stories from the present. The CG is outright bad and resembles something from the late 90's. The traditional animation, on the other hand, is a little better. There's still an awkward hiccup here and there, but I enjoyed the style. However, even in HD, it lacks any distinguishable "pop."
As much as I'm willing to give Aftermath a second chance after playing Dead Space 2, it's hard to imagine any significant uptick in quality from what is presented. Unless you're dead set on knowing everything there is about Dead Space's mythos, you can skip on Dead Space: Aftermath.