Faithful to the Source Material.
I like shooters and action games, but not so much that I buy a ton of them. I just mix them into my usual array of RPGs and open world games. I got this game specifically because it was 40k. I'm a big tabletop gamer. I play the 40K RPG and have played some Warhammer miniatures in the past. I love the setting and the world that Games Workshop has built.
With that said, I was very pleased with Space Marines, mainly because they really did the source material justice. The people making this game must of been fans. All the trappings of the 40k universe is represented in a faithful way. I really enjoyed that they worked the Inquisition into the story.
My love for the setting aside, it's a pretty fun game as well. The weapons are different and they give you reasons to switch between them. The melee combat is a blast. I love how the game works healing, which requires you to do a kill move on a creature in melee to regenerate health. There are some jump jet levels that are also enjoyable and a nice change of pace. Overall, the game just really fun.
I do have some gripes with the game. It was too short for my tastes. Might want to buy it at a discount. Also, while there is a variety of enemies, mostly you fight space Orcs. Of all the bad guys in the 40k setting, I find them the least interesting. Towards the end of the game, you move on to some other creatures, so it's not all Orcs.
So if you're a 40k fan, you can't miss by playing this game. Even if you're not, it's still a very good game.