Beneath all the wonderful graphics and lovely warhammer theme, this is basically a hack-n-slash arcade game. It is extremely linear, with little or no room for adventure or interaction with other characters. You are forced down a linear path to kill various enemies using various weapons. That's basically it. There is no sense of adventure or exploration at all.
This game is great fun to play, on a simple and sastisfying 'kill everything in sight' basis - not a 'lets explore this area and see what's here' basis.
I just think nowadays even the basic hack-n-slash games need a touch of adventure about them, and some room for exploration. I was looking forward to exploring the Space Marines' world as much as undertaking their mission. Would it have been so hard to include a little stragegic decision-making, as well as just hammering the 'fire' buttons?
Having said all that, the combat is awesome, if a little repetative at times.
For a thrid person shooter, however, it has some rather annoying restrictions. You cannot jump. You cannot crouch. You cannot carry more than 4 weapons. These are basic things but they do bring the gameplay down a notch when they are missing.
I haven't played the online multiplayer yet because I am too addicted to the single player campaign.
Space Marine is definitely worth buying but don't expect anything more than a linear hack and slash affair.