Just finished skimming through my recently purchased PDF copy. Largely pretty great; I particularly loved the symbiote feats, and the Mocking could fit into just about any campaign.
I did notice a couple of errors: columns two and three of page 21 managed to get transposed somehow, and Wormwood appears at a glance to be missing a CR. (It's there - 20 - but the art covered it up; I had to highlight the text to find it.)
I was mildly disappointed by the Star Beasts, actually. They just don't feel nearly as impressive as it seems like they should; only Wormwood even approaches the sheer terror factor of a (comparatively) common great wyrm. I understand the design intent - clearly the star beasts are meant to provide opponents for a range of campaign levels. Even so, the decision to do it that way robs them of the epic feel I was hoping for.
All that said, it's still a pretty great book. Nice layout and pretty pictures go a long way towards making an enjoyable read in my book, and there's a fair amount of utility to be had here as well.
So, as a first impression: 4/5.