Space Marine is the very definition of mediocre.
Walk through a series of empty rooms in a rather bland industrial complex. You'll come across ammo crates and wonder: am I supposed to have been shooting my gun? Is there a bug preventing enemies from spawning? Eventually you come across a room and there might be three enemies to shoot. Or they might swarm out of a given location. You fight for ten minutes. You hop onto an elevator and then run through some more empty rooms. Meanwhile, the game will randomly slow down and chug, at random points that do not correspond to anything visually impressive.
The pacing of Space Marine is so incredibly bad. I couldn't help but continuously wonder WHY they needed me to run through bland environments for huge chunks of time. The way fights and encounters setup is just so bad. Except for a few jetpack sequences, every fight has to be approached in one way and it's usually just... look enemies in front of you. Kill them. There are plenty of exploding barrels but they are rarely setup in a way that makes them enjoyable to use.
It's so frustrating because there is clearly some fun to be had in this game. I dig the Warhammer universe and it's on full display here, with Titans and Orks and fabulous looking Ultramarine armor. Wading into the melee with a thunder hammer or power axe is great fun. The bolter sounds incredible - the impact is well done.
But everything else is mediocre. The 'twists' of the story-line are obvious and the dialogue is purely functional, rather than inspired. The pacing and level design are, as I mentioned, nothing short of atrocious (seriously, I cannot emphasize enough how bad it is). There are some inexcusable performance issues. The color scheme is muddy, a real shame considering Warhammer's long history of painted figurines. The hidden 'extras' which take the well-established form of radio messages / audio logs are boring, as if no one put any effort into them.
I did have fun, sure, but Space Marine was definitely one of those games that I beat because I'd already bought it. I was happy to move on.
(I suspect that Space Marine will be on sale during the upcoming season and many will be here, looking for the question: is it worth it? I would say... no. There's just so many better games out there. Get Bulletstorm instead.)