Great game to play...ONCE

WarHammer 40,000 is a great universe. Grimdark without being ridiculous. 7-foot tall genetically-engineered Space Marines slaughtering aliens to protect humanity.
So it seems a natural choice for an FPS.

In Warhammer 40,000: Space Marine, you ARE a Space Marine Captain, you ARE one of those 7-foot tall genetically-engineered ass-kickers in ill-fitting Powered Armor.
You start out with only the basic Bolter (Pistol), but you soon get a Heavy Bolter (Machinegun). You can get other weapons for your other two slots (You're limited to 4 weapons instead of the 2 that have become Industry Standard, that's how badass you are!), as the situation necessitates.
There's a flamethrower for close-range combat, but it does all its damage in one burst instead of spraying out like you'd expect. Still fun to see enemies on fire, or if they're armored, their armor glowing red with heat.
There's a Laser Cannon and a "Stalker Bolter" (Sniper Rifle) for long-range work and later the laser cannon gets upgraded to a Plasma Cannon!
And your cute little Bolter Pistol eventually gets upgraded to a Plasma Pistol.

You are also able to dish out serious pain in melee combat, Dynasty Warriors style. In fact, almost EXACTLY like Dynasty Warriors in space. Your knife will soon be permanently replaced by a Chainsword, but you can trade out the Chainsword for a Power Axe, and later the almighty Thunder Hammer, which can do AOE ground-slam attacks!
(Sadly, while wielding the Thunder Hammer, you can only use the Pistol and Heavy Bolter, but still a good trade for your awesome swing-and-pound attacks!)
Your Armor regenerates, with a hexagon "Force Field" animation when you get hit or when your shield recharges. IMPORTANT NOTE: Your Armor regenerates, your Health does not...BUT, you can Stun enemies and Execute them to gain health, with moves inspired by Brutal DOOM. And yes, they do vary based on what weapon you have equipped. From Slice&Dice with the Chainsword to beheading with the Power Axe to knocking an enemy down and either stomping its head in, or slamming the Thunder Hammer into its head--which creates an AOE shockwave, pushing other enemies back and giving you some breathing-room!
There are HORDES of Orcs (And later Demons), and the fighting gets frantic, fast, and furious!

Special mention must be made of the Jetpack you can find that allows you to jump to the high-ground--and perform diving attacks on enemies that are already there!
However, again, you get limited to only Pistol and Machinegun Bolter!

The enemies are well-represented, from the cowardly Gretchins (Goblins by any other name) to Squigs (Suicide-bombing lizards), plus the bog-standard Slugga Boy melee Orcs to Rokkit Boys (Rocket-launcher wielding enemies...Yes, it hurts, a good test of your dodging abilities!), Nobs (Noble Orcs who have whatever weaponry they feel like, but usually decide to charge into melee--and they can pick you up and bodyslam you!), and even a suitably hard encounter with a Warboss. (Orcs cheat, so when the Warboss senses he's losing, he calls in a ton of reinforcements to distract you. Later, you get Chaos Marine enemies (Who have evil copies of most of your powers and weapons), and Demons, who have the annoying habit of blur-dodging out of the way then sidling up next to you and slashing into you.

The story is not original, but well-done version of one done hundreds of times before: You are responding to an Inquisitor's call for aid, to rescue him and protect an experimental Power Source he developed...using the power of Chaos. Chances are approximately 100% that this will go horribly wrong, even WITHOUT Orc invasion! There is no interaction to the story. Yours is not to reason why, only to do or die.

The environment is random, as befits a bombed-out warzone. Rubble, girders, and pipes will just HAPPEN to fall in a way that makes ramps or stairs to your next objective. Mortars or incoming "ROKS" (Orc troop-transports, looks like a ball of rock and mud and wood) will just HAPPEN to break bridges that would make your progress too easy, and all the other Broken Bridge Tropes are in full effect. It keeps you on a very linear path, it is NOT an Open-World game! It's easy to get lost in the game, but if you take "too long" the game will put a glowing fist icon where you need to go. You just have to figure out how to get there!
There are rewards for going off the beaten path, but only for those interested in lore and backstory. Instead of tapes or record players or whatever, here you are collecting skulls with one glowing red eye. They contain the last words of those who went before, some even before the invasion started. Everything from fathers mourning dead family members to overworked and unappreciated Techpriests (Technical Support for the far future). And of course, military logs, even the Inquisitor's bragging about his research!
If you play the STEAM version of this game, you get an Achievement for collecting them, otherwise, this is just backstory. The skulls do not make any audible cue, no special crosshair or anything appears on them, and not every red light you see is a skull's eye, so this hunt gets frustrating fast. I don't consider it worthwhile to go back and try to find them all. If you do, more power to you. 

http://mrantifun.net/index.php?threads/warhammer-40k-space-marine-trainer.508/
This trainer is the only free one for this game that I know of. It provides unlimited Health and Ammo--and skips reloading, which helps make the fights a little less frantic, and even MORE fast and furious with you being able to constantly fire the Machinegun Bolter, or fire either of the Special Weapons as often as the animation allows! The Special Weapons go from Too Awesome To Use to actually useful tools in your arsenal!
Being invincible makes you FEEL like the heavy Powered Armor, as bulky as it is, is not just for show. It feels good to power trip and plow through nearly infinite Orcs and Demons, don't you think?

Overall, it's a great game, but only worth playing once, unless you want to try it on other Difficulty Levels, or find all the skulls. It's very long, but stays linear.
The enemies stay the same, the story stays the same, and you start out weak and only gradually find the upgraded weapons. If you could keep your weapons and go through it start-to-finish with all the weapons, or if it unlocked an Open World mode, that would change everything, but as it is, it's a "One And Done" game.