A fun, blood-thirsty riot that doesn't quite reach it's full potential
First things first, those foreign to the universe might want to do some reading up while they play the game. Space Marine doesn't do the best job of giving any background story, and new comers will be lost when they start hearing about adeptus astartes and emperors and xenos taints. It will all make perfect sense to veterans though, and effortlessly feels like it could be another chapter in the Imperium's never ending war.
Graphics-
Space Marine's art style perfectly captures the grim atmosphere one would expect from 40k. Smoke, fire, bolter and laser trails, hordes of enemies coming down on you, spews of blood splashing everywhere, a lot of things are usually happening at once in this game. Instead of photorealistic graphics, Space Marine goes for a more artistic style, but that fits the chipped power armor and stubby bolters just fine. It looks great across the board, and runs great on PC even if you don't have a cutting edge system. Throw on a little film grain effect, and Space Marine looks like a gritty world where victory is never certain.
Sound-
The sound is solid, but doesn't exactly stand out for the most part, the music is typically completely forgotten in the middle of an ork horde. The weapons on the other hand are great though, from the staccato hammer of a bolt pistol to the angry buzzsaw grind of your chainsword, all the weapons sound beefy and quite powerful.
Gameplay-
The gameplay is Space Marine's greatest strength and weakness. At first casual glance it could be mistaken for a Gears game, but it's more a pseudo thirdperson shooter/action game. You can carry four guns at a time, but you can really only choose two because you will always have your pistol and trusty Bolter. Orks tend to attack in hordes of melee heavy troops though, so combat usually switches to hand to hand very quickly, which is easily done with a click of the mouse. And this is where Space Marine's weakness comes in. While the battles are fun and brutal, that's really all there is to it- shoot and slash until everything is dead, then do it again. Combat is generally easy until the second half of the game where things get ramped up and you'll probably die more, but your tactics will basically remain the same. The lack of a cover system is defintiely felt at some points as well.
The Final Word-
I would give Space Marine a 7.5 for a variety of reasons. Namely I feel that it doesn't take full advantage of its universe. While it feels like it's 40k without a doubt, there is no epic feel or characters or story- it's all quite shallow and predictable. The gameplay itself is also a bit shallow, though undeniably fun. So, Space Marine is an easy reccomendation for a 40k fan, but for someone new to the series wanting to get in on the action, this year's Dawn of War 2 Retribution would probably be a better choice. Space Marine is a great starting platform, but just falls short of any kind of epic potential Warhammer 40k deserves.