Summary: The DK-19 rubber eyecup works as advertised. It is easy to install as a replacement for the standard viewfinder eyepiece on your D3 and D700 (and I assume the D3X as well). Be careful not to get fingerprints on the _inside_ of the glass piece that covers the viewfinder, which is reused from the standard eyepiece. I find that the DK-19 both improves my view through the viewfinder, and perhaps even more importantly for me, reduces the scratch hazard to my eyeglasses.
Pros: Easy To Install, Protects eyeglasses, The Price Is Right
Summary: I purchased the DK-19 for my D700 on the recommendation of another photographer. I wear glasses and always seemed to be smashing my face into the view window on my D700 and was also getting stray light in the finder, making it hard to see what I was doing. The DK-19 installs in minutes following the instructions and sits out enough that I can put my eye to the cup without smearing the display panel and it reduces stray light significantly.
Pros: Comfortable, Cuts stray light, Easy To Install
Summary: Installed the DK-19 on my Nikon D700. The installation was quick, but you will probably want to read the instructions first. The eyecup is a huge improvement over stock. It helps block out extraneous light, and adds comfort when shooting all day. It does fold back against the camera body if you don't want to use it for some reason.
Summary: Keeps stray light out of the viewfinder. Very helpful in low light shooting. I love eyecups, I only wish Nikon would include one with every camera!(3 grand for a camera and they don't give you one...
Pros: Durable, Enhances viewing, Good Value, High Quality
Summary: Contrary to what I've been told, the DK-19/DK-17 combo fits just fine on my F4 (of course the locking feature isn't supported). This leads me to believe that it will fit on earlier F models as well. I didn't give it 5 stars because the DK-19 and the DK-17 are both way overpriced (big surprise).