I put an adhesive screen protector on the LCD of my XSi when it was brand new (that was about seven months ago). Since I barely even used the camera without the protector, I really can't say whether it had much affect on the appearance of the display, but I don't think it did. I'm not sure I believe that it reduces glare. The surface is still very reflective, and there is still glare.
My main concern was protecting it against scratches. I did manage to scratch a screen once and was forever annoyed by it. This protector is definitely doing its job on that count and has not developed any scratches of its own. I'm also more at ease when cleaning the display with whatever fabric might be handy while shooting. So overall I'm happy with it and would do it again.