I have the Martin Field screen protector on my DS Lite. They sell a set for the DS and DS Lite. The package comes with a top and bottom screen protector, but I don't see the point to the top screen. I don't ever touch it, so why bother? I'll just save the top screen protector and trim it down if I ever need to replace the bottom screen protector. Here's the link:
http://www.overlayplus.com/US/modeldevice.asp?category=Nintendo
As for the protector itself, it's wonderful. The backing is ever so slightly tacky so that it sticks to the screen, but not sticky enough to leave residue or to be a bitch to remove. The protector is also absolutely crystal clear. I used to use plain old vinyl when I had my original DS, and it is nowhere as clear as the Martin Field. The Martin Field also is very slick, so there is no stylus drag like you would get with vinyl. Once you get it on, you can't even tell that it's there.
However, since I've only just got it on, I don't know what kind of longevity it has. You can remove and wash it, and some reviews say that they last just as long and probably longer than any other protector out there.
My only complaint is that it can be a pain to put on without getting dust or air trapped under it. You can use a piece of tape to lift it off at one corner and, using the same piece of tape, to remove any trapped dust on the underside, or to set it down properly to remove the air. I had to do this a few times and I guess that in bending the protector several times, that the extreme left edge of the protector doesn't sit flush on the screen. You can't notice it when the unit is on, and it doesn't seem to affect the touch screen sensitivity.
Another problem is that it is a bit more expensive than the Hori screen protectors. I bought a set for my DS Lite and another for my nephew's DS, and the total ($12.99 for each plus quick shipping) came out to $31.
Overall, I really like it. It's crystal clear, no noticeable stylus drag, and it's supposed to last a really long time. Just beware that it can be a bit of a job to put on without getting air or dust underneath, and it is a bit more expensive than other similar products. I would recommend it to anyone, but if you can't afford one, I suppose that plain vinyl is better than no protector at all.