The Planescape setting is an incredibly complicated one, and yet the average character living in it would know quite a bit about how it works. Some kind of Player's Guide to Planescape was necessary to give starting PCs an idea of what their world was like -- to fill the role in this setting that a player's own experiences fill in games set in modern-day Earth.
The Planewalker's Handbook is intended to cover basic questions about the planes, almost like a FAQ in narrative form. It describes the planes in a general sort of way (though it does a better job than that silly players' pamphlet that came in the boxed set). It gives you the mechanics of portals and planar spellcasting. It talks a bit about the factions and major NPC races. It provides some kits for the DM to throw out. It outlines, once again, how belief works.
Once you get past the @#!$@$# cant and actually start to read, the book is friendly and conversational, and seems to realize exactly how huge an infodump is required to get new players into the planes. It still feels like a textbook at times (the Priest Level Loss by Plane and Magical Item Bonus Loss by Plane charts at the back are painful to look at) but it's still a good resource.
The question is whether an infodump is the best way to get this information across, or if the Planescape DM might not be better served by requiring all players to start their first characters as primes and teaching them this information gradually. That's something each DM will have to decide for herself.
(Side note: it has nothing to do with this book per se, but am I the only one who finds the "Acheron, plane of combat, is a giant void full of bouncing dice" joke idiotic?)