If you're a fan the Aliens movies, R.E.4, Doom 3, or just the sci-fi and/or survival horror genre, this is a must-play.
So yes, the graphics are very impressive as is the audio, which is very creepy and immersive. But where Dead Space really shines in the gameplay. Similar to survival horror games like the Resident Evil series, resource management is definitely part of the game. It may not seem so at first as the game gently shows you the ropes for the first few levels so you get the hang of the mechanics before it starts throwing wave after waves of tough, smart, and fast enemies at you.
Don't be fooled by their zombie like looks though - these monsters can think. They won't walk into traps. They'll constantly flank you and they'll use the airducts (which they can access, but you can't) to sneak around and get the drop on you while you're distracted fighting others.
What makes the gameplay unique in Dead Space is a combination of the weaponry and the tactics required to succeed. Issac (your character) is an engineer, not a soldier, so it's appropriate that most of his weapons are actually engineering tools (there is only 1 firearm in the game). From the laser cutter which is your first weapon and will last you the whole game, to a hydrazine torch (flamethrower) to a force gun, etc. each weapon has a unique use. There are more weapons available than you can carry. You can carry a maximum of 4, but can switch out at any store and you're nver that far from a store. Figuring out which weapons to use against which enemies is part of the strategy. The other part is the way you fight. Going for head or body shots like in a normal shooter is useless here. The only way to defeat these necromorphs is to cut off most or all of their limbs. This makes combat more precise than most shooters. It also makes fighting multiple enemies very challenging since both accuracy and speed are a necessity. Fortunately the game gives you a power which can slow down time in a small area, but it must be used sparingly as you only have a limited amount of it.
Another power you have is a telekinesis like ability. Unlike the time power, this one has infinite uses. Unfortunately, although it's fun to use, it's mainly used in simple puzzles like picking up and plugging in batteries. It can be used in combat, but most items, other than explosive canisters, do so little damage that it's hardly worth using. More could have been done with this power in terms of gameplay, but it's a minor issue with so much else to keep you occupied.
I also want to quickly mention zero gravity. This is the best use of it I've seen in a game. You will be in zero-G several times in the game. In fact, zero-G is the only time in the game where the telekenesis power fully comes into play with some interesting puzzles. Zero-G combat is hectic and you'll need to have your head on a swivel since enemies can, and will, come at you from the left, right, front, back, and (unique to zero-G), above and below.
The story is pretty interesting, though it doesn't really pick up speed until near the end of the game. Before that, similarly to Doom 3 and other games, you'll mostly learn about the disaster that stuck the Ishimura by picking up video and audio logs of its crew. However, I strongly suggest that if you are intersted in the story, that you download all 6 of the animated comics available for free on Xbox Live. They tell the backstory of the game and lend the story much more depth.
You owe it to yourself to play this game if you are interested in the science fiction and/or survival horror genres. Or if you just want to play a fun 3rd person shooter with a different, more precise take on combat, some amazing looking (and acting) monsters, beautiful environments, and that sense of immersion that only a combination of great art, music, and technical skill can achieve.