While all of the images are impressive, I found the hair rendering to be the most interesting. It's very tough to get it right, that is, realistic-appearing. As presented it's very close to that ideal.

An attribute of hair that makes it hard to render is color. At least on my monitor the "blonde" sample seems a bit too reddish. (Blonde is really sort of a dull green.) Another factor is color variation, not only the result of lighting, but also intrinsic to hair growth. (Commonly hair at its "roots" is slightly darker/lighter, and there's natural randomness among individual hairs.)

I hope that doesn't sound like picky criticism, it really does look very promising. Portrait artists I know would empathize with the challenge of convincingly depicting the hair of human heads.