I didn't find that the shield made much of a difference at all. The shield basically protects you from the front... but 99% of the time I was killed by someone who surprised me from the rear or side. I was rarely worried about the guys in front of me.
I suppose the shield could help when making your way through closed doors, but I usually stand to one side of the door, wait and listen for any triggered terrorists approaching from the other side (they like to walk out just after you reach a door), and if all is quiet I'll pop the door open while maintaining cover.
I started using the shield but found that I prefered the speed and flexibility that I had without it.