I use DK-17M the whole time on my D7100 (by adapting a DK-22), and DK-21M on my D40x and D5100.
Bjørn, you are possibly referring to DR-3 the right angle finder?
I find it quite useful for night sky imaging high in the sky, not sure about other purposes as aiming the camera looking down get a little weird, copy stands come to my mind as one application. There is no magnification with this one.