Huh, I didn't realise that if you 'Preorder' you get the first 1/3 of the game now, like an early access with a fixed release date I guess. Just played the first 3 levels. Holy sh** this game is so much fun, even more than Quake, with it feels very similar to.
It's quite a bit easier than Quake though, I think. I've been playing on the hardest difficulty level, which only gives you 1hp, and I've only been stuck a few times, and only once for any significant period of time. It would probably be a lot more difficult (and frustrating) if you didn't use quicksave. I'd probably recommend on the 2nd highest difficult if you've played through Quake before.
But seriously, get this game if you enjoy old-school FPS at all.