I've struggled with the same issue.
For me, I think it's all dependent on which weapon you're using.
If you use a shotgun, you have to get ridiculously close to hurt someone.
Otherwise it's all about bursts to the head.
Still love the game.
My Games of 2011:
The Legend of Zelda: Skyward Sword
Elder Scrolls V: Skyrim
Super Mario 3D Land
Uncharted 3: Drake's Deception