The first time you play it, Space Marine will probably be one of the most exciting FPS you will have played. The graphics are fantastic, the voice acting is the best I've ever seen, the game play itself is satisfyingly violent and the scenery if fantastic. And the overall atmosphere is what you would expect from a Warhammer 40K game.
However, there are a few problems. The chief among them is a total lack of any replay value, beyond simply trying it on a different difficulty level. You can pretty much try out all the weapon combinations on the first go, and there isn't much room for tactics in this button mashing game. Yes, you can collect 'servo skulls' that give you more background on the story, but in my game they seem to be buggy and sometimes don't even show up at all.
Another problem would be, as other reviewers have said, there isn't any room to explore. It's all very linear and forces you down one path. So really, the world around you is little more than eye-candy, and the makers could have done MUCH more with all that fantastic scenery.
Still, as FPS's go, Space Marine is one of the best in recent years, it's fun, it's gory and atmospheric, and I can tell you that I will never, ever get tired of killing orks.