ORC is fun for me for 3 reasons:
1. I'm an RE fan but not an RE fanatic. It's fun to see RE 2-3 elements used (not abused) in a co-op non-canon RE zombie game. This also helps with the following two reasons.
2. It feels a lot like Left 4 Dead. The gameplay itself is in no way similar but minimal story, zombies mixed with "boss" zombies and teamplay focuses make RE:ORC feel like it's in it's own world with L4D, which is a pretty nice world if you can get into it.
3. If you can get away with comparing L4D and RE:ORC the one thing ORC has on L4D is the passive and active abilities. Each character has two passive and three active abilites (but you can only pick one active ability to use in any mission) that range from cloaking to incendary ammo to turning zombies into allies (and crimson heads too) to body armor and many, many more.
Plus you get to use the Samurai Edge and silenced guns.
There's more, good and bad, if those points sound good to you I would say go for it but you might want to wait for the price to go down (or until April 11th) to pick it up. Unless you really, really liked Prototype. Fan Fact: my speed runs of RE:ORC take longer than the story of Prototype 1.