I got the alcantara version of these pads several days ago and I have a couple impressions:
On the "overview" page they advertise these with "slow-rebound memory foam" But, while they may have memory foam, I do not find it to be particularly slow-rebounding memory foam, at first glance they were barely distinguishable from regular foam.
They aren't angled earpads like the pads for the TH/TR X00 series, which I guess makes sense because the stock TH-900 pads aren't angled, but they do taper a bit at the top (or bottom depending on which way you rotate the pad) where the seam is, which is a bit odd but I guess it's supposed to be what is mentioned in the description that they "contour" to your head.
The alcantara itself feels nice, I wonder if it would be even better if they made perforated versions.