Yes, it's worth it. The game is great in SP AND co-op. In fact, it's probably better to play the game single player first just to enjoy the spooky bits by yourself. Note that beating the game in SP unlocks the ability to play as the other character and he's got a completely different play style. Great supplement in co-op and still fun on his own. This game was just plain fun. You can bet your behind it'll get cheaper but it's a great game all the same, even if you just want to play alone.
That said, it is short. We are DEFINITELY talking 4-5 hours maximum. I've beaten it twice though and it hasn't gotten old. There aren't that many linear shooters I can say that about. It's just a solidly produced game. I really enjoyed it and recommend it for fans of the previous FEAR games.
Oh, and forgot one other negative, the score system in the game is tied to persistent level up unlocks of more health, more slow-mo/spirit meters. This is utterly stupid as it means players are gimped unless they work through the grind. Not DECISIVELY so nor is it a dealbreaker but still irritating.
EDIT: Worth noting, I believe this game was made by a different studio and was meant to be the FEAR2 before Monolith got the FEAR rights back and made their own true FEAR2, making this one FEAR3. The difference in design philosophies shows and would be an interesting study for those who like that sort of thing. Really, FEAR2 was a huge storyline step into the crapper from FEAR1, and FEAR3 is an even bigger step in a different direction, not necessarily the crapper. I think the writing in FEAR3 is better than FEAR2's, but we are definitely talking a DIFFERENT game thematically than the other FEARs. It's got the same sort of "shoot for a while. Spooky stuff for a while" pacing and atmosphere, but it definitely doesn't feel like it naturally flows from the previous FEARs. It's still a well-told game story in its own right, though and not quite as, in my opinion, dreadful as FEAR2's story was.