Got it, am happy with it; nice to see some updates on Planescape material, although I've been generally wishing that they would have revived Planescape in it's true form. Unlike some others, I was more disappointed with the 3E Manual of the Planes: it was such a a let down in the wake of all the 2nd edition Planescape material, that to see the planes revived without the voice and feel of PS was a disappointment. The Planar Handbook helps bring a bit of that back, though.
I also picked up the Book of the Planes from Mongoose not long after; actually, a very nice book and emminently compatible with the MotP and the Planar Handbook. Yes, I believe my next campaign arc shall involve a new Tale of the Society of Planar Cartographers and their endless quest to map the planes.......
On the crunch-factor of splatbooks waning: yes, I feel that vibe, too. I have moved past the BoED, Draconomicon, Savage Species, and Arms And Equipment Guide as just not worth it. These days, I find I am only really using the Complete Warrior, Complete Divine (sort of), and BoVD (and the Planar Handbook/MotP, of course). UA gets a bit of mileage.....not too much, but enough to keep it around, just in case.