Finished F3AR and have to say I thoroughly enjoyed it. I'm a bit of a FEAR fanboy though, played the first couple of games pretty endlessly. I'm happy to call this a worthy sequel and a nice apocalyptic trilogy-finisher. It's not particularly scary, although I did jump a few times and made an embarrassing yelping noise that the missus heard from the bedroom. It's certainly more creepy and tense than the 2nd game, but lacks the impact of the 1st.
Gameplay-wise it's pretty smart, really tight shooter mechanics, an occasionally useful cover system and the joy of slo-mo. The slo-mo doesn't have quite the same kick as it used to (there's a lot less audio and visual feedback to it) but in general it's a very nice looking game, but without the showyness of 2 (playing 2 now it feels like there's vaseline all over your eyes with the amount of motion blur and HDR lighting on show). The ranking/leveling system is fun too, and suggests that co-op is probably the best way to play the game so I'm looking forward to trying that.
Read a couple of pretty negative reviews of this, but I'm really happy with it.