Quoting Movemodo's Dead Space: Extraction review:
"Dismissing Dead Space: Extraction as a stripped down version of Dead Space is a mistake. Extraction is a fully fleshed out title that brings the story behind Dead Space to life in great detail. The unique play style lands the game as one of the premiere on-rails shooters available on the market with its intense gameplay and deep character interaction. For a limited time, Extraction is free with the purchase of the PS3 version of Dead Space 2, or available as a standalone title on the PlayStation Store."