I liked it. Better than L4d in my opinion. Also KF was in development before L4D was even heard of. If anything is copying anyone, its the other way around.
It's not bad for the price, i spent a couple weeks playing it consistently and still fire it up once in awhile. Kills a couple hours at a time.
Actually they just added some new weapons that made it a hell of a lot better. just be aware the graphics arent amazing and the voice acting almost makes you laugh a little. Beyond that the music is great, zombies explode, and weapons are fun.