We faithfully used a Roadmaster Guardian on our trip to Alaska and the Jeep windshield still got two huge cracks due to rocks being tossed above the shield and to the side. Additionally the leading edge of all fenders (and the air dam) looked like they were shot with rock salt from a shotgun.
The Guardian did protect the area immediately behind it; however rocks would ricochet off the Guardian and fly into the rear cap. We must have 20 or 30 dings in the cap thanks to the 'Guardian.'
After Alaska, I took the Guardian off and haven't used in in two years. I think I'll use it as a target holder when I sight in my deer rifle each fall.
I am absolutely convinced the best protection is the skirt like Fakrwee mentioned.