The art is a mixed bag of really good, ok, and one artist I simply don't care for. However, the Steve Prescott art makes me smile. I've been a serious fan of his stuff since my exposure to his work in Shadowrun. Good stuff and I'm glad to see WotC using him more. Very cool.
So far my only nitpick about the material in the preview is that while it mentions the Quasielemental citadels of the Doomguard, it tries to put them in the cardinal elemental planes. That's fine, except by that measure:
Sealt = water, NOT earth.
Exhalus = air, NOT Negative energy plane.
Crumbling = fire. They got this right.
Alluvius = earth NOT air.
The whole point of it all was to have them corresponding to the 4 elements as they spiraled down into negative energy and were broken apart by entropy. As it stands, the 3e version has 1 of 4 correct, and 2 on elemental earth. The rule of 4 and rule of opposition are broken. *Yugolothy frown*
Still looks cool though, some of the text is literally straight out of the 'Factols Manifesto'.
Of course, I'm still waiting for the book to come out so that Sigil will get it's 3e update beyond the MotP. I'm going to be particularly happy about a few things therein. *twirls razorvine tiara*