I've used them on a Lumix camera, a Nexus One, an LG g-slate, and an HTC Sensation (with a contoured non-flat screen)....
They definitely make the critical difference in outdoor full (tropical) sun visibility! (I can actually use the camera and N1 and frame the pictures for a change
) Indoors, there is no noticeable difference in brightness or color accuracy, and there is zero graininess even when I read the small screens with magnifying glasses.
On the Sensation, which has a profiled gorilla glass screen (perimeter is raised so the glass doesn't touch when you put it face down), I did have to trim it down to 1mm inside the illuminated area of the screen to make it stay perfectly flat.
Out in extremely bright sun, the image will obviously be a few clicks darker than it would be indoors, but you CAN see it clearly in situations where it would be
totally illegible without it. I was also pleasantly surprised to find that I can still read the screens while wearing polarized sunglasses.