Bought my n900 when it was first released here in UK and only just bought a pack of screen protectors (7 for about £3 from eBay) about a week ago. Generally careful with it, but still got 1 or two tiny scratches on the screen.

Screen protector is ok - does spoil the look a little, and I may or may not be imagining a slight loss of responsiveness, but on the whole well worth the money - should stop any future and more serious scratches.

Just for comparison, my Palm Tungsten T3 which I've had for a fair few years now has a fairly badly scratched screen (mostly around the Graffiti area), and I've been equally careful with it. It's just general wear and tear. Much better to wear out a screen protector than a screen! And no, it's not just cosmetic - the T3 screen is definitely less responsive in the more scuffed areas.