I have now tested a little (2 hours).
At first I compared the differences in quality settings between PT20 on Vanilla, min and max settings.
Max runs about as fast as Vanilla but looks a bit better. Min runs over 40 % faster than Vinilla. For example, on an RTX 3080 you get 45 fps in UHD DLSS performance at min compared to 31 fps as in Vanilla or max. This option makes pathtracing playable on many more graphics cards. If you turn up the quality setting the PT area light shadows become finer and the GI effect becomes slightly stronger but between medium and high it is hardly noticeable. I didn't see any difference in many scenes. Medium also runs much faster than high. On min settings some surfaces were visibly darker due weaker bounce light. This comparison only relates to PT20.
Within PT21 the quality differences between the settings are almost imperceptible and you gain about 10 % fps between high and low. PT21 can only be a little bit faster than PT20 Vanilla and therefore it is not as accessible for user with slower graphics cards.
In PTNext only the vanilla setting seems to work properly. The other quality settings are missing some lights and the resulting shadows.
It's exciting when you suddenly have so many PT options instead of PT on and off.
From my point of view this would be an intersting topic for DigitalFoundry
@Dictator. Compared to most other current games and topics this pushes the technical limits of what is possible. Also PT20 min makes this game playable on many more graphics cards and even AMD ones.