I just finished the Operation Catalyst campaign for CrossfireX in about 2 hours and 48 minutes (based on Xbox gaming tracking)
Like I said before, it's very mediocre. I was curious how much the game changes after the first hour and it doesn't change at all. Storytelling is wack and actually reminiscent of Remedy's past titles (of course) with lots of strange visions/flashbacks. Gunplay is a mixed bag; gun handling and shooting feels nice and 'juicy' with the 'Combat Breaker' (bullet time) ability but shooting at enemies does not feel good at all (actually reminds me of the Uncharted games where the shooting sucks ass).
Something I like about the shooting is that the bullet time ability is a bit different than your average slo-mo shootout. It does this thing where the action "speeds up" and "slows down" based on whether or not you are currently shooting; when you aren't shooting, the world moves in slow motion but when you are, it's like the world speeds up to show the immediate effect of your shooting so there's this amplified and exaggerated effect of enemies reacting strongly to your shots. I think there is also a bit of "aim assist" while you're in this mode because I've noticed that my gun snaps directly to the nearest enemy in my view once I kill the enemy I was shooting. So there's this kind of "staccato" rhythm to the kills while in bullet time that feels nice. Maybe a closer analogy is watching a film sequence with every other frame removed so that it feels faster and more frenetic (see the opening of Mad Max Fury Road do this). Thankfully, the bullet time ability recharges fairly quickly so you can try to spend most of your time in this mode. If you do get really hurt though, your health actually regenerates quite fast. I think I've only died like once in this game and that was because I tried to go Rambo into a large group of enemies; it's still very possible to pull off and survive and I think that might be the best way to play it knowing that you can heal fairly quickly and use bullet time frequently.
Other than this bullet time ability, the game is just bland. Run of the mill run and gun military shooter with limited weapons and not much varied gameplay (there are sequences where you snipe people from a distance like an escort mission to spice things). If you are curious how Remedy does an FPS with limited resources, then check it out. Just set your expectations very low.