However, it is at times too much like Manual of the Planes. I get it that the Paizo folks really enjoy a lot of the old D&D tropes, and that they also believe that their core market is based around folks who weren't interested in migrating to Fourth Edition because it changed too much stuff. But there was a very powerful sense of déjà vu while reading this... because much of it really isn't all that different than the Great Wheel cosmology. It's slightly simplified; the Golarion cosmology has an outer plane for each full stop on the nine point alignment system, while the Great Wheel has these bizarre filler planes in between each of the full stops on the nine point alignment system. Also, planes like Heaven, Hell, the Abyss, etc.---many of them are very familiar even outside of a D&D context, and they're pretty much... exactly the same as they are in D&D in that case, or only very minorly tweaked. A few had minor name changes, but otherwise felt very familiar. This focus on alignment is unfortunate, I think, since alignment, after more than thirty years of gaming, still doesn't make a whole lot of sense, and is more a constraint than a useful tool, in my opinion. The best thing that can be said about it is that using alignment as a guide creates a situation that greatly resembles the Great Wheel cosmology. For better or for worse. I never use anything like the Great Wheel cosmology personally, but I know plenty of folks really dig it. The Great Beyond also features a set of inner planes and transitive planes that are... very familiar to D&D players. Although with a few nifty tweaks. Unfortunately, the same problem that these planes have always had in D&D carries through here. What in the world do you actually do in a place so inhospitable and empty as, say, the Negative Energy Plane? (negative energy? Think about that concept for a second anyway. What in the world is negative energy?)
Where the book excels, rather, is where it does something new. D&D has always had a long, storied history of exploring devils and demons, the "lawful evil" and "chaotic evil" fiends respectively, it's never really had a good concept for the in-between fiends, the "pure" evil or "neutral evil" group. They had the daemons back in the day (which was confusing since that's a little bit like saying color and colour are two different concepts... when you can't even tell them apart by saying it and of course they mean exactly the same thing being, in fact, the exact same word. Daemon is merely the British spelling for demon.) This concept never seems to have completely taken off, so in the Second Edition era, they were renamed and expanded upon greatly, remade as the yugoloths. Although this excited a small population of hardcore Planescape fanatics, otherwise the yugoloths also seemed to have been a concept that struggled to find fertile ground in which to grow. Green Ronin sensibly came up with a concept for them based on the seven deadly sins of famous Catholic doctrine (and Brad Pitt/Gwynneth Paltrow gritty cinema interpretation), although unfortunately they used the word daemon again. Paizo seems to have come up with yet another concept, basing them, thematically, on the Four Horsemen (of the Apocalypse) and making them a band of nihilistic fiends, arguably amongst the scariest in the lower planes now. So, good show there.
There's a few other really good ideas strewn throughout the familiar landscape of an abbreviated and expurgated Manual of the Planes (it's even organized more or less the same!) including the demi-plane or "other" plane Leng, ripped straight out of Lovecraft's The DreamQuest of Unknown Kadath. While not explored in this book very much (one of the last Second Darkness adventures gave this detail, if I remember correctly) the Paizo guys have re-used a lot of the demon heirarchy that Erik Mona used when he wrote Green Ronin's Armies of the Abyss, including a nearly complete demonic "pantheon" based, mostly, on Judeo-Christian demonology.
I guess in general, I'd say this product, like The Manual of the Planes that it so strongly resembles, suffers from being a book that's difficult to actually use in play, but which is interesting to read, at least. However, the fact that it also so strongly resembles the earlier book makes a good half of the book feel almost superfluous.