A review by a long time 40k player and a long time gamer.
This is the part for non-fans. This game is not for you. The plot and reasons you should care lack context in every conceivable way. Without knowing what a "Warboss" actually is, or a "Daemon Prince" you really have no idea why you should care. They don't really even bother to explain what a Space Marine is. The game's world is designed with the expectation you already know this stuff. You won't connect with any of the characters, and you'll find the story utterly bland and uninteresting. If you spend a year talking to fans of the tabletop universe, you'll start to get some context as to why the events on the game are a huge deal.
Before I get to the stuff for the fans, I'll continue on the technical merits and demerits of the game for the non-fans.
Here's the good: the shooting is visceral and rewarding, the melee is gory and violent, you even get some awesome blood spatter cake your powered armor, the visuals are great, and the controls are tight and responsive. Its got the aesthetics down to a T, and the shooting down with it.
Here's the bad: the game is way too short for a AAA title. I finished my first playthrough in about 6 hours. If I'm spending 60 bucks on a title I expect a whole lot more for my money. Luckily I only rented this game. If you're a fan, wait for it to go down in price before you drop your load on it because you -will- leave feeling completely ripped off.
Here's where I get really irate about the game. This title was sold for months to fans of the 40k universe on the "amazing" Intimate Brutality system. In other words, it was sold on the melee combat. The melee combat sucks. More specifically, the melee combat is incredibly well executed if the game would allow you to fight in melee. Basically, you fight in melee you get shot to pieces and then chopped to pieces in short order. The only way to regenerate your health is by performing executions. The executions are in-combat cinematics that look spectacular. The problem is you're locked into these executions while you're performing them and take full damage from all enemies. So let's say you're in a mob of Choppa Boyz, and you execute one, the other 10 around you take advantage of their free shots on you. Or let's say you've caught one behind a corner and someone throws a grenade at you while you're trapped in the cinematic, you're dead or going to lose the health you just gained. The game needs immunity during these cinematics and regenerating health bars, or at least some inventory first aid kits. The little power shield a la Halo just doesn't cut it.
Your character is also way too soft. There have been times where I've been instantly killed from full health and shields to empty from a Chaos Marine with a Plasma Cannon or 2 Rokkit Boyz hitting me simultaneously. Given how hard it is to earn health back there's really no excuse for the fragility of a Space Marine Captain.
The squad and enemy AI especially in relation to each other is horrendous. There have been times I duck behind some boxes to regenerate some shields only to find that my squad mates are out of cover in the open staring at the enemy not even firing their weapons. The fun part is neither are the enemies. Everyone is waiting patiently for me to regenerate shields and then when I break cover all the enemies focus on me instead of my squad.
All-in-all the game is mechanically proficient. Its not so game breakingly hard that you can't get around the terrible health system, and its still fun to play. From a gameplay perspective its fully functional and works rather well. Its just not going to be winning any awards and you'll probably forget about it a month after you've finished it.
The game also ignores the 4th pillar of gaming: story. There's some character development for Captain Titus, but his character comes out only slightly more interesting than a stump. Fans will be quick to point out "he's a Space Marine, he's supposed to be one dimensional". That doesn't really hold water in a gaming industry that demands story. The massive and bloody 40k universe isn't developed as a character either, its merely alluded to with the expectation the player already understands what they're talking about. There are far less full motion video scenes than they could have used, the story just generally fails on all levels. Really, any chapter of Space Marines other than the Ultramarines would have been more interesting. Ultramarines define the vanilla marine. Space Wolves have refined senses of humor plus their genetic advantages, Dark Angels have that Catholic guilt going on plus the Fallen Angels, Black Templars have had their home world destroyed by the Orks plus their crazy Emperor's Champions, there are any number of better suited chapters that could have built a stronger story, or stronger characters.
Now here is the part for fans. WARNING: SOME THINGS HERE COULD BE CONSTRUED AS SPOILERS!
From a tabletop perspective the game is amazing. When the Warboss fights with the Daemon Prince the fight goes as one would expect in tabletop, the Warboss holds his own and survives and the Daemon Prince struggles with the Warboss both being on about even footing with each other. You could really tell when designing this game they didn't want to make players of the armies alienated by the perceived weaknesses of their important characters. Rokkit Launchers are Space Marine killers, Flash Gitz are incredibly tough and annoying, Nobz take a freight train to bring down, etc. Chaos Marines are on an equal footing with your player character, much like tabletop, your Captain wins through mostly an equipment superiority.
The weapons function like tabletop as well. Melta-guns pretty much destroy anything they touch, Storm Bolters will tear up Orks but do less than nothing to Chaos Marines, Las Cannons/Plasma Cannons will be incredibly effective against vehicles and Chaos Marines, and so forth. The weaponry pays massive lip service to tabletop.
A lot of the problems I have come with the ending. Again, spoilers. Basically, a Veteran Tactical Marine sells out his company Captain to the Inquisition based on blind superstition. Any SM fan out there knows there are channels for this kind of thing and the Inquisition would never be involved, it would be handled within the Chapter. Space Marines also aren't psychotic superstitious of their Battle Brothers. They trust them implicitly, especially if they are a higher rank. If this kind of thing ever happened, the person who sold out his battle brother would be murdered in his sleep. It would have made more sense if the tactical marines standing around shot him to death as the thunderhawk flew off instead of him standing there looking like a kicked puppy.
The other issue is that after the Inquisition threatens to harm the Captains company and the Imperial Guardsmen under his command for the campaign, the captain buckles. Any Space Marine would have told the Inquisition to "go away" (using nice words) because Space Marine Chapters don't answer to the Inquisition and the Inquisition threatening a company of Space Marines is basically a declaration of war that the chapter would be happy to oblige. I should note I play Space Wolves in tabletop so my opinion of the relationship between Inquisition and Chapter is different than other players. Basically, the game makes Space Marines out to be the lap dogs of the Imperium. They bark when told to, they kill when told to, they die when told to, by any member of the Imperium higher than an Imperial Guard Lieutenant. So while they capture tabletop's mechanics, they don't capture the history of the political power and sway of the Adeptus Astartes.
In reality the game deserves a 7.0. However, Relic entertainment has never done a 3PS before and this is their first attempt at breaking into the genre. Given the technical proficiency, smoothness of the graphics and gameplay generally, the game gets an extra half a grade for being a solid first attempt.