I used D400 only when I was a student, 20 years ago: exactly when the current version came out. Back then I didn’t know what to look at, and my exposure and development were not optimal, so I can’t say anything first hand.
From reading -for years- hundreds of posts in Photrio, Leica Forum, FADU, largeformatphotography, rangefinderforum, Photo.net, etc., this is what I conclude:
Delta400 is a bit slower than HP5+: lots of users report horrible grain growth and blocked highlights at EI400: that’s common mostly with D-76/ID-11. It seems at 400 in MQ developers D400 has a contrasty/pushed look. I’d say this comes many times from HP5+ users who know by experience the lowish native contrast of HP5+.
Its slightly lower speed makes a lot of users say EI200 is correct. I’d say the film was called 400 for DD-X use, the same way TMX and TMY are not really 100 and 400 in D-76 unless we push development a little, but those two reach box speed in TMaxDev with great tone.
Most users reporting great tone and great image structure with D400 use metol developers, like Perceptol and Barry Thornton’s 2 bath developer, exposing at 200 or 250. For sunny scenes wet printed, diluted Perceptol seems the best option for controlled highlights and best grain.
All this is nothing new, and not really far from HP5+, but as D400 doesn’t have the same “different speed grain layers” as often called, it’s not a film equally good, for uprating, as HP5+, or at least not to the same degree: HP5+ can be beautiful at EI3200 for wet printing overcast scenes, while I’d say D400 at EI800 is fine, but at EI1600 probably it’s a better idea using HP5+.
I’m not very interested in D400 for pushing or for street, but I’d like to see its best version for 35mm wet printing for mixed scenes rolls. Xtol doesn’t produce beautiful sharp grain with D400 when it’s wet printed, as far as I know, but if it does, it would be good to take a look.
Of course, scanning is a different field, and software/virtual sharpening can create things that are not in the negative.
Especially, it would be cool to see if D400 has its own look, and its own grain too, as beautiful as those of Tri-X when it’s correctly exposed and correctly developed for wet printed sharp present grain, but with high resolving power and great clean tone despite its present grain in the whole 35mm frame.