Tried the DG-2 yesterday during my experiments, but it makes the view very narrow and small, and that's a real waste with the D600's big viewfinder - I plan on trying it for macro and other close focus uses, though, as that seems to be its intended use.
The DG-2 also sticks way out, and has that metal piece too, and will likely loosen in your bag just when you don't want it to do that. That's been my problem with any of the rubber eyecups, which I do like - I finally glued my rubber eyecup onto my D7000 after chasing after it one too many times as it rolled away, and it finally does not come off all the time.
What Nikon needs to do is to make a rubber eyepiece that fits over the DK-21C Diopter +5, that I have been playing with . . . . or, a Diopter correcting eyepiece with rubber around the edge, like the stock one on the D600, only a bit larger (wider by only a tiny amount - a centimeter?).
But, the DK-21C Diopter +5, does match my lousy vision in my good, left, eye, and it's really nice to get a clear view of the focus without having to pull reading glasses out of my pocket and lose the shot while I'm busy doing that. Oh the wonders of aging!