Dead Space is completely competent action/horror. It's the perfect example of one company observing another company's success and aping it perfectly. What elevates Dead Space above the Cold Fears of the world is that it isn't satisfied with just copying the thing that inspired it. No, actual thought and craftsmanship went into making sure every part of the game was tight, tuned, and almost entirely hassle-free.
This is the video game equivalent of a well-written beach novel or competently-made summer blockbuster. It's pretty and violent and even clever now and again. Lacking originality isn't a mortal sin, and it shouldn't keep anyone who likes action/horror games from giving this one a spin.