I am impressed by this great shooter
To start with, if you are looking for ultra realistic and fully fleshed out campaigns and multi-player, then you need to look at a more mature property then 40K. But if you want a taste of what the 41st millennium holds in store for humanity and witness how humans can turn soldiers into immortals, then this game fits the bill in spades.
To get the details out of the way; solid story in the campaign. Great action and control of the main character. The camera is the biggest weakness: there is too much manual movement required and the camera does not auto focus on your target when the swarm grows and you in melee. The endless waves of Greenskins also needs an update with some more variety.
To someone who is already versed in 40K mythos, the campaign will be far more meaningful. If you idea of a Space Marine is from the movies then you will miss out on the subtlety that is included right from the opening scene; very understated view of a computer screen showing a conversation that has enormous meaning only to those in the know. And that is my biggest gripe about this game; I doubt that many new fans of Warhammer 40K will be made just from the story in this game. THQ did a much better job in their strategy games of setting up the background.
However I have to say this is exactly what my imagination always indicated how a Space Marine would behave and act and move. In a word the Space Marines are epic. I am eagerly awaiting the Multiplayer add on's to increase the options and I am keeping my fingers crossed on a sequel.