I felt the slo-mo added nothing to the game; it didn't add a tactical advantage since everything moved at the same speed.
Actually, your crosshair moved at normal speed giving you a tremendous advantage in aiming. Also, since everything is moving slower, it gives you MUCH more time to think about what you're going to do. For example, if you saw someone raise their gun at you in slow motion, you'd have more time to decide if you were going to shoot first, run, throw a grenade, etc.