This is in reference to planetary observations, which is where the Naglers come in. They are among the best eyepieces out there. I used them especially during the 1988 Mars opposition, using a 10-inch refractor. (The ALPO Mars recorder was often with me).
The one disadvantage I found was the amount of glass.
I recall some guys coming in the observatory one day and discussing eyepieces; I was pretty firm that Nagler's were the best ones there are, but they insisted Zeiss made better eyepieces.
Years ago I "rubbed elbows" with Al Nagler, who is known as a pretty good guy personally, though I never really met him.
Without having personal experience with Zeiss eyepieces, I cannot say. There is a brief but good summary here:
An Amateur's Guide to Observing and Imaging the Heavens
By Ian Morison