I flipped through my copy, especially the races and classes/prestige classes and I'm pretty dissapointed. I think that the amount of useful information in the book that will actually get played is almost negligable. I think I could photo copy 10-15 pages out of the book and have more than I'll ever use (or see used in my groups). I think the book is aimed for people who are playing games that are almost entirley on the planes.
I also think so design decisions were not so great. They could have added several monster progressions (there are plenty of ECL5-15 outsiders), maybe even progressions from outsiders from Exalted Deeds or MoP (reprinting them in the back wouldn't be all bad). I think they should have dumped the Mephlings and put in the Gensai (don't think they have been published outside of FR), maybe with a nice 3.5 overhaul and actually balance them out. Maybe a little more native outsiders would have been nice ("Ooops, sorry bob, you can't fight this bad guy 'cause he has Prot. from Good up" "Ooops, you were booted back to your home plane with no way to get back, roll up a new character." "Ooops, no Raise Dead for you, roll up a new character"). I personally disliked the alternate level thing for the base classes, I just think they are largley underwelming and set a bad design precident (isn't this why we have PrCs instead of this stuff?). The PrCs themselves were dissapointing, I only saw 2 worth while ones (no one I know plays Planescape anymore, so their value for flavor is pretty much lost). The feats section seemed pretty bad, a lot of those feats just seemed abusive, and sadly they left out cool background feats (like the racial ones in Races of Faerun) that are race specific (like Aasmiar specific ones, etc). Finally, the big granddaddy was the planar touchstone things, what a cool idea! Unfortunatley they wasted like 1/5th of the book (between the descriptions, spells and feats relating to them) on this thing very few people will use. Spending a feat to gain an ability you may one day get to a place and get, on top of that the 8 ranks of Knowledge (planes) makes it restictive. Maybe if it only required 2 ranks and character level 6 or somthing, and you could take them after you been to one.
Wow, sorry for that rant, I guess I disliked the book more than I thought. It wasn't all bad though, a few of the races were Ok (including the monster classes) and there are some Ok spells. At the end of the day I guess I was expecting more stuff that could be used in more general (non-planarcentric) games. I think they should have either made the book much more accessable to more players (suggestions above) or made it into "Planescape 3.5" and got it over with.
I'll go over it once or twice more, give it another chance, but I think it's going back or being resold.