I put on two GGS protectors - one each for my D4 and D800. They are the regular ones, not GGS III. The quality and fit appear fine, although with the D800 you have to dispense with the plastic LCD guard as this will not fit on top of the GGS protector. I guess this is understandable as it is effectively as substitute. Unfortunately I did not realise this until I had put it on. Although according to the instructions it is possible to prise the protector off again, I decided not to risk it and didn't!
The D4 doesn't have a plastic guard so there were no issues there. As I say, they seem to fit well (there are two for the D800, back and top, and three for the D4 with the addition of the lower window displaying ISO etc). No troubles with fitting, although it requires a steady hand with the camera positioned solidly between the thighs as it is so sticky you wouldn't want to try and lift it again and re-align. It went on easily with no bubbling or blistering. The D4 protector one is definitely bespoke for the D4 as it has a small (unobtrusive) line of text stating D4 on the black lining of the glass.
It doesn't seem to make the LCD any darker although it must lose a percentage of light passage through the glass. They are very thin. According to the bumph "achieves more than 90% of transmittance (higher than plastic LCD screen protectors)"
Altogether I was happy. I paid £12.95 for the D4 and £11.49 for the D800. The D4 LCD is meant to be scratch resistant and hard-wearing anyway but for the small cost, I think it's a worthwhile investment and one I am happy to make.