Lotsa funny info going on here and in the Leica world in general.
APO is beneficial for Color where all three primary colors match at the same focus point. The people and the weekend-reviewers who want to sound expert about it and who recommend the APO for the MM are not to be listened to. They give very weak arguments.
Also, the APO is not a no-brainer. Just look at a HCB print, or McCurry print, for example. The last thing you'll even think about it all this technical junk. And no, none of their images would have benefited from a better lens. Not even Capa's famous blurry/soft image of the soldiers. All the magic is in the eye connected to the finger.
At last, I have always been very disapointed in the Leica crowd in general as they're usually proven that they don't really know much about photography in general. Take the 50 Lux Asph for example: It used to be maligned because it was "too perfect", or "too clinical", which just isn't true. That particular lens is a top performer on all aspects and it has plenty of character. Plenty. Exactly the opposite from "clinical", a term that I suspect was simply coined by a couch photographer who couldn't' afford it and was looking for a reason to love his old glass a bit longer. And now, it seems that good old "modern 50 asph lux which is too clinical" has suddenly become old glass to the Leica crowd. Quite funny.
The 50 APO is a status symbol. Your girlfriend loves Gold and big Diamonds? You love the 50 APO. It's as simple as that.
At last, my take on the APO is very simple. It is a status symbol. And probably a very solid performer. A total waste on the MM. And a total waste if you want to improve your photography. And I don't believe in diffraction. It's true in theory, but in practice it vanishes. This is why even cheap glass picks up more and more details as the sensor has more and more MPs. And it's all about viewing distance, anyways.
The 50 APO? Only for fondlers. Capa wouldn't have benefited from it even if he had run for his life, case proven by his famous shot. Or Ulevich's Pulitzer image, where he almost got killed. No APO or ASPH would have been relevant at all. A Pulitzer just doesn't doesn't care. I believe the APO, beyond any of its "perfections" will show a unique character.
Will it be worth it? If the resale value doesn't drop, it's always worth it, even at 25,000$. Simply logical.
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A great book that I recommend to all those who think that photography is about the equipment, be prepared to be smashed: Moments: The Pulitzer Prize Winning Photographs by Hal Buell