Excellent at $250
I'm not much on audiophile but I own sennheiser momentums and a 15ish year old pair of ath a900s. The most immediate difference to those really old ATH A series headphones is these are much lighter and smaller. The clamp force is much stronger, I get a much better seal. They changed the design to include no swivel up and down on the cups. It fits me well and seals, better for me, but I've heard that some people have issues because the fixed angle isn't right for them and it leaves room in the area above the ear. Luckily my head is fat enough.
My ath a900's have brainwavz xl pads on them since the old ones wore out long ago, and I honestly don't remember how the stock ones sounded. With the better seal, I think the ath a1000z is immediately the more "right" sounding headphone, as that seal is pretty important for closed back headphones. It's more in your face, there's more isolation.
Other than seal, the biggest difference so far is the bass is much tighter and less fat sounding in the a1000z, but the tuning seems to be reasonably close to the a900's. The a1000z is less sibilant, but both are still known to be a little bit sibilant if you listen to meh recordings. I've only listened to a few pop tracks so far so I'm only really picking out the super obvious stuff.
For gaming, the a900's light seal lead to some confusing imaging issues sometimes, so in that respect, the a1000z is much better.
One downside is the cable is kind of crappy. The ath a900's did not have removable cables, but they were thick and braided. The cables on the a1000z are not as premium feeling.
I think ATH makes quality products; this one is made in japan, and considering how long the a900's lasted me, I'm hoping these will be similarly well built.