I've only tried a couple of screen protectors so far (I've only had my n900 for about a month); I had a miserable time trying to apply them without embedding bits of dust beneath them or misaligning them to the screen, but I've had one on for a few weeks now without any perceptible reduction in brightness or sensitivity. This one is a "Lexerd TrueVue"; one thing this brand provides is a couple of fingers from a plastic glove, allowing you to touch the sticky side of the protector without getting it dirty. This made it much easier to realign the protector!

I dont know whether a screen protector is absolutely necessary, but I do enjoy using the stylus; and the combination of "long pointy instrument" pressed against "flexible plastic surface" just makes me nervous. Maybe it is completely unnecessary, but it makes me feel a little better...