Having finally seen the book myself, a few first impressions:
1) The massive white space in the layout of the book is annoying to no end.
2) The ravaasta picture is just damn cool. And so is its writeup as a reimagined monster. It's not the same arcanaloth, but then again it's not the same cosmology, so outside of any preconceptions carried over from the original, it's a spiffy monster.
3) The planes vary from decent to very well written. I could tell which sections Rob Schwalb worked on, because I liked them best as far as themes and general elements I like that he has a tendency to include.
4) It's not a bad book. But it's very clear that my idea of and favored design approach to the planes is often antithetical to the design stance the 4e writing team used in working on the book, and in making their favored changes upon concepts that they converted over from the Great Wheel cosmology.
5) The artwork is a very mixed bag for me. Some pictures are just spectacular (the ravaasta, Dispater, etc) and others look amateurish (grazzt and the maralith). And then there's the fact that they continued the 4e trend of recycling 3.5 artwork. In just the first look, I found artwork that originally appeared in the 3.x books Sandstorm and Lost Empires of Faerun. It's jarring and tacky, and I have to wonder why WotC has been continually doing this, especially since they're putting books out in a more selective manner at the moment. It's their time to showcase their best work and emphasize a style for 4e, and I find myself going, "Nice art. I rather liked it the first time I saw it, several years ago when it was illustrating something else entirely. Now it's just jarring to see it crop up again with the serial numbers filed off."