I just bought an InvisibleShield for a Vista HCx after reading threads like this one full of rave reviews, and I'm pretty disappointed with it. Applying it was easy-- no bubbles and very quick compared to other protectors I've used, and it's nearly invisible when using the backlight-- but it makes using the device with the backlight off almost impossible.
I don't know if it's the adhesive, or some sort of polarizing property of the plastic, but it's very difficult to read the display with only sunlight now that the shield is applied. It almost looks like the bottom of a lake when sunlight shines through the ripples in the water-- lots of patches of glare and shadow.
I have a similar shield for my cell phone that does not have this problem, but they don't make one sized to fit the unit, and it's not an easy square shape to cut. Anybody know of any alternatives? I really don't want to waste battery life with the backlight, but I don't want the screen scratched either.