Depends on the screen protector you get. For example, I have an InvisibleShield (one of the thicker ones) on my n800 (yes n800 not n900) and the touch screen is easily still usable until you try to draw nice pictures on it. That's my main problem with it, if you want to use it for really precise stylus actions a thick touchscreen is going to throw a wrench in that real quick. Granted I'm not much of an artist to begin with so it just might be my terrible drawing skill.

But I've noticed occasionally it'll shift a few pixels over from where I intended to draw which then makes it fun cause then you have to erase it.

Edit: I wonder what would have happened if that video used an American coin that has ridges.