Just opinions, no facts here. Jump to the bottom for performance review.
I recently acquired some antique kellners from the 80's. From reading S&T's from that era, it appears that eyepiece choices for the average amateur were the Huygenian, Ramsden, Kellner, Ortho, and Erfle. Of these 5 only the ortho appears to remain a popular choice. They sell fast here, that for sure. Guidebooks generally advise against Huygenian and Ramsden ep's, rate Kellners as the minimum acceptable,and praise the orthos and Erfles.
Today, where does one find these ep's? Huygenians and Ramsdens can be found at the auction site, usually as dinky ep's to go with dinky scopes there. I have never seen these ep's advertised in my time astronomical (1997 to present)
Kellners are still around. Meade MA's, Celestron SMA's, Edmund RKE's, and the 3-element, 2-inchers like the Celestron Elux are all Kellners according to various internet sources. Also the crosshair ep's in finderscopes.
Orthos and Erfles are still made by University Optics and Antares, but are rarely seen. Who's buying these?
The Kellners and Orthos of the pre-80's have been replaced by the plossl. The plossl explosion of the 80's may have been started by Televue in 1980. Compare a 1980 magazine with a 1984.
In the attached photo are several ep's. The top row are the Meade MA's, old Japan style and later Chinese type. The Japan is quice the nice ep and superior to later models. It has better coatings, larger eye lens, more eye relief, and better construction overall. Note the reflections.
Bottom row left is a generic silvertop 25pl and a 25 Celestron Kellner. The reason for the plossl's ascendance is quite clear: larger eye lens, wider fov, sharper in fast scopes, and (cheaper to manufacture?)
The bottom right are two identical Japan 40 Kellners, one is an Optica b/c and the other a Celestron. In the 1980 mag there were at least 5 different advertisers selling the same ep's.
How well do they work? Very well. In a 178mm f/15 the Celestron Kellners gave sharp, bright images with a narrow fov. The Japan Meade MA was right up there with the plossl, but with less fov. I also have an Edmund 28mm RKE. Its sharp in the middle, but the outer 1/3 is not.
In 80mm f/6 and 50mm f/4 the Kellers gave pleasing, sharp views with minor edge distortion. My 6mm ortho wont reach focus in these scopes, cant get close enough. The RKE is only sharp in the center, the outer 4/5 horribly distorted. Next week I plan to try these ep's in a 317mm f/5.
Its fun to experiment with these fossil ep's, they work very well. I have a curiosity about eyepieces that are no longer available. Makes me appreciate the Panoptics and Naglers all the more.