Being a devotee of Planescape, I was looking forward to this book, which was supposed to be a reunion on Planescape writers, and comprising of brand-new exciting planes for the planar traveller to enjoy.. Sadly, it was a disappointment. The first chapter was pretty good, with its discussion on planar connections and planar travel. However, when he got to the planes, each were lacking. None feel like they should be a plane; they all felt like they should be a planet that a Spelljammer campaign might visit. Relit a sun which will warm up the plane and allow life to once again flourish? That would work better in Spelljammer.
A place where LN xorns discourage travellers so they won't take away the valuable minerals that are as common as dirt on the plane? That would work better at a Spelljammer planet?
A place where spider-like beings have to fight an invading, powerful ant-like beings? Once again, that's not a plane, that's a planet for a Spelljammer crew to visit.
Add to this a lack of crunchy bits like new feats, spells, etc., and you have a mediocre book. If it was a Spelljammer book, it would rate a 4, or may a 5 due to a dirth of material. But as a planer guide, which has no planar flavour whatsoever, it ranks its 3, which would have been a 2 save for its worthiness as an unintentional Spelljammer supplement.