I received mine today!
First the good:
The build quality is much better than the previous aura hd. No creaks - it just feels solid. The back feels very nice too, much more like a paperwhite: that slightly texturised rubbery feel. Both make the aura h2O a pleasure to handle!
Feels like quality.
Contrast is much better. The blacks truly are blacker. With and without light! Big difference. This makes the fonts look very, very nice.
The background is lighter too, but I doubt it is the advertised 50 procent. More like... 20-30 procent if I had to guess. But you can certainly see the difference. Is it as white as a sheet of paper? Hell no, not even close. But it is certainly whiter when compared side to side to the aura hd. Overall it is a good, solid improvement.
Unfortunately, in my specific case, the bad:
Not pleased with the comfortlight on my unit. The light is much more sickly yellow-greenish than my aura hd, I thought it was supposed to be soothing white (ish)? Mine is not even close to white except at 100 procent but who on earth is going to read at night with 100 procent frontlight

Also: some mild white blobby lightning effects and much more shadow effects on the bottom half of the screen. My 'old' aura hd has a much more even, uniform look when it's lit. So I'm returning this one and crossing fingers for the replacement.... I think I received one with DiscomfortLight.
We'll see.