It comes off as a bit abbreviated, but i think it is probably Wotc's best 'fluff' product.
Unlike the Forgotten Realm setting books, which overwhelm the reader with a great deal of basic detail that may stroke a certain kind of anal fantasy fan (I may or may not be one of these) the right way but which is of little interest to most games, the fluff here is actually zeroes in on the adventuring material; its very broad, but very suggestive. It even ties the 'hook' into the game's skill mechanics.
Some think the prestige classes are overpowered, but I tend to disagree. The special abilities, though flashy, tend to come at a large cost, either explicit or oppurtunity; if anything, they tend to point to why earlier (pre-Theurge) prcs were underpowered. There is a lot of fluff surrounding these pcs as well, though it tends to be a bit crunchier than in other setting books, with the aforementioned skill mods, benefits, and the like.
Some also seem to think that the fluff has skewed too much into a 'high magic' direction, more akin to FR. Of course, they are looking at it as a cohesive ultra integrated 'world'. I look at it more as a basket of suggestions, the actual tone to be created by my players and myself. In the interest of diverse combat challenges and scenery, most plot hooks would have to involve mechanics that have a 'fantastic' sense of fluffiness. If the mix of elements in the hooks were constrained by the need to make the immediate reader feel that the settings 'tone' were being adhered to, the hooks would come off as repetitive and boring in play.
So, in conclusion, if you are a hardcore fan of the setting in itself, than you might find the criticisms of the book compelling. If you are a fan not so much of the setting, but of the elements of the setting you can bring into your game, than the book is quite good for that, though more dm focused than player.