I've had previous touchscreen experience with first a Psion Revo, then HP iPAQ H5450 & hw6915. The Revo had a curious surface finish and during years of use, never got scratches or anything. Both iPAQs on the other hand had very vulnerable displays, hell, the one and only time I've let someone unexperienced handle my unprotected device it received a very visible p-letter shaped scratch on the dislpay. That made me really paranoid with my N900, until I got a protector for it.

I used a Zagg Invisibleshield for about half a year, until a bad installation (too close to lower edge of the display it seems) caused some dirt to start getting under the edges. I took it off and replaced it with a Celly protector.

The touch screen felt certainly more responsive with the Zagg protector, for two things:
1) it was much thinner, more like a film, than the sheet-like Celly. Of course, the Celly was much easier to install.
2) it has a "sticky" surface, hard to describe, but it felt easier to use the disaply with it. While the added friction made it easier to press with fingernails accurately, the was making sliding movements more difficult.

I now have two Celly protectors (4 of these cost the same as one Zagg), not sure what I should go for next. My girlfriend's father has a really great screen protector on his N900, I'll try to find a chance to ask what brand it's from.

Bottom line; Zagg feels great but is expensive and started accumulating dirt under the edges (also under the edges of the cut holes for camera, speaker, etc). Celly makes the touch screen less responsive but feels very durable, also doesn't look as cheap due to being "clearer"
I've heard much good about the Vikuiti protector, but have no personal experience.