I actually really enjoyed this game. I belong to the niche market that both enjoys the Warhammer 40k universe to the point of reading dozens of novels and also enjoys many third person action games. If you don't then Space Marine may not appeal to you as much and you may want to try before you buy and may indeed wish to skip this review, which assumes you are familiar with both.
Initial Marketing suggested that Space Marine would be a game that mixed Gears Of War and the Warhammer 40K license together but surprisingly, Space Marine actually ended up closer to God Of War than Gears Of War is several key areas.
In the campaign there are seventeen levels spread across five chapters, which can last about twelve to fifteen hours and which feature a good variety of enemies, including: Gretchin, Choppa Boyz, Shoota Boyz, Rocket equipped Boyz, Two variants of 'Ard Boyz Squiggs, at least three variants of Nob, occasional Stormboyz and airbourne vehicles, occasional variants of turret towers, occasional weirdboyz, traitor guardsmen, three or four variants of traitor marines, pyskers, Bloodletters (its not much of a spoiler as they're on the box art) and a few others.
There is also a good variety of weapons to be employed in the campaign; which except for grenades, come in three forms: Combat, Ranged and Exotic. You can carry one Combat weapon and up to four Ranged weapons at a time, wheras Exotic weapons are taken in combat from mounts and replace all other weapons until they run out of ammo or are discarded.
Weapons include: Combat Knife, Chainsword, Power Axe, Thunder Hammer, Bolt Pistol, Plasma Pistol, Bolter (In this game however it works as an SMG, rather than the destructive explosives-weapon from the mythology) Stalker Pattern Bolter (Sniper Rifle) Plasma Gun, Melta Gun (In this game, essentially an overpowered shotgun), Storm Bolter, Las Cannon, Heavy Bolter, Plasma Cannon and a few others including a game-specific triggered explosives launcher.
The pacing is good, slowly introducing new weapons and perks (in the form of purity seals which improve your `Fury,' which is a mechanic like Rage Of The Titans from God Of War) and letting you get used to each one, before moving on and eventually allowing you to chose between them.
There is no cover system to speak of, so the game works more like an old PS2 game in that regard. Health is gained by pressing a specific button in combat in an almost similar way to God Of War/Devil May Cry type game, only without Orbs and only upon a specific button press (but with a regenerating shield bridging the gap between Health Hunting and Regenerating Health)
It probably goes without saying, but being a Warhammer 40K fan helps you to enjoy the game a lot, as there are lots of little references left unexplained just to get your brand recognition flaring and causing a little spark of enjoyment should you be in the know. Some are more obscure than others (Skittarri for example won't be familiar to you if you only know the brand from videogames) and most of these references are found by collecting Servo-Skulls.
The Warhammer 40k Universe is very well realized and both looks and feels fantastic, however a lot of the plot, dialogue and voice acting is a little too hammy if you prefer the Dan Abnett/Aaron Dembski-Bowden-Bowden dark and semi-realistic version of the universe, as opposed to the all-orks-are-cockneys and space marines are pontificating poets version.
The gameplay itself isn't absolutely perfect either if I'm being entirely fair; for example there are only two boss fights, only one vehicle section and some of the audio logs are a little too short and uninteresting when compared to some of the games that pioneered the format. Additionally, there isn't much that hasn't been seen before in other games and if you play a lot of third person games, it may seem a little stale. Put simply, the gameplay is a blend of tried and tested mechanics from lots of other games that, while very fun, doesn't particularly innovate.
The multiplayer is fun (as long as you get a code and don't miss out on half the content) if nothing very new, there are only a few modes and very few maps at present and nothing here hasn't been done before either, but serves more as a nice little addition to the single player experience, rather than a selling point in itself.
Overall; Space Marine is a strong and more importantly fun game that will keep you entertained for a few days and should definitely be considered if you can find it at a low price, especially if you fall into the aforementioned niche market of 40K fan and 3rd Person Gaming fan.