Finished it, expect 10-12 hours on normal and while I don't think I'm any slouch when it comes to FPS, I managed to die a few times towards the end. I'd give it 9/10. A game that knows what it wants to do and does just that and not a bit more. If you aren't sure, wait for sale, which is what I usually do, but this time I bought it right away and I'm glad I did.
The gameplay is really well done, it managed to remind me of almost every classic FPS at different points, Half Life, Doom, NOLF, ... The story has a few dead ends and it's definitely not something that will make you keep playing, but the background story is really well executed through cutscenes and the ending is what you expect, but not what you want to see. I went in expecting braindead shooter, so I was pleasantly surprised, but it's not Bioshock or Half Life 2.
My biggest complaint is probably the boss battles, because mechanics are always the same, shoot the glowing spot then the weak spot, every single time, with random ammo and health dropping in the arena. You have these absolutely gorgeous arenas that are empty, I was really hoping for one level in the shadow realm where you just cut your way through swarms of demons, but nothing ever came
Well, except like 3 minutes in the last chapter... The writing will be hit or miss, expect references and the word "wang" used on every possible occasion which I didn't mind, but I expect some people will come here to say it's terrible.
Weapons are fine, I ended up using every single one of them, except the flamethrower, because you have to be pretty close and then you might as well use the katana... The crossbow is kind of shit, it has really low rate of fire, but I didn't get the final upgrade, so that may improve it a lot. Rocket launcher ends up being the goto weapon for solving difficult fights, along with the warlord head, which is the classic "overpowered energy weapon you never find ammo for" which is elegantly solved by it not having much ammo, but being dropped every other fight toward the end from one of the most difficult enemies. RPG elements are well executed, but because the combos have so long wind-up time, you usually have time to throw like one spell at the beginning and then maybe some emergency healing.
Level design puts all CoD clones to shame, even if they are linear, you can always see places you just went through and how are you moving around a bigger complex, basically like Half-Life.