Experimenting with DK-21C Diopter +5 on my D600, and just read half dozen old threads here about playing around with eyepieces to see what works best.
So far. . .
Pro - I can see through my better eye ( left) (-7 correc., yes, I'm blind as a bat) without glasses, reading or otherwise, to focus clearly. I like this. . . I think.
Con - The stock, non-correcting rubber eyepiece won't go on over it (as pointed out in the '09 threads, for a different camera). As I wear contacts, not glasses, no worries about no rubber there, and the D600 viewfinder gives a big view which I wouldn't want to lose. As I don't shoot in foul weather (or, is the rubber there just for easing contact with glasses?), is this a good compromise?
Any better suggestions for those of us in our Golden Years, or for anybody with lousy vision, at any age? I know I could see an optometrist for something custom. . . . Anything off the shelf that I should know about?