F.E.A.R. 3 is an average, and unremarkable, horror first person shooter. If you have ever played a fps then you'll be familiar will everything that F.E.A.R. 3 has to offer, if you have never played a fps before... well that kind of odd and remarkable, and you probably shouldn't have your first fps experience with F.E.A.R. 3. This isn't to suggest that F.E.A.R. 3 is a bad game, but more to the fact that the game lacks polish.
It has some nice things going for it, the campaign gameplay varies enough to stay fresh, there is co-op (which I didn't get to experience since none of my friends have this game), and the two playable characters feel slightly different from each other.
However, the game has significant things going against it. For starters, it isn't scary, which is a problem, considering that it is a horror fps. Second, the campaign itself drags on for way too long and often feels like it is stretching things out, just to fully use an environment (a la, if we spent months making a haunted department store, then the player is going to see every freaking room in it). And lastly, the game's story is very narrow and straight to the point, so if you haven't played the previous games in the series, you'll not going to understand everything that is going on, and even if you did, you're still not going to understand everything.
If you play this game under ideal conditions (co-op with a good friend that scares easily) then I would image that this game would be great, however, that isn't going to happen for the vast majority of people, so this game is not worth it.