Im near the end now, and to be fair the second half is much stronger - I still roll my eyes every time the Lambent turn up to spoil the fun, though.
It may sound weird, but it really reminds me of Left 4 Dead, the way that you're up against swarms of, essentially, zombies most of the time.
I'm sure as Keef says, that it's fun to play co-op, but if you ask me, gearing the campaign for four players has kind of spoiled it slightly as a singleplayer experience. The dynamic has changed as a result, and the firefights lack a lot of the tension of the previous games. As it's so easy to get revived with 3 NPCs fighting alongside you, I rarely die even after cranking up the difficulty.
Yet another singleplayer game compromised due to the multiplayer! Don't get me wrong, I enjoy the odd game of co-op and horde, but for me the meat of the game will always be the singleplayer, and in this respect Space Marine was much more enjoyable.