Gaia, which I own, is in my opinion a sufficently solid book.
I like the fact that the setting is very vast and has space to accomodate so many flavors and themes of oriental-aesthetics-adorned fantasy, from Vampire Hunter in moth, to Lodoss-like in Helenia, to vikings, pirates, samurais, arabian nights, musketeers...
I also like the history of the continent, the "secrets" stuff, some of the setting flavor really is fntastic.
On the other hand, the book has its flaws. A good chunk of the informations on the single nations is, to put it clearly, bland. I don't care about the "atlas" information on flora, fauna, mountains and lakes or the most important trade of the place, unless it's going to impact the game someways.
I make a clear example: I open a random page on the book: I get page 135-136. The Twilight Brotherhood window is simply great and oozes style. On the other hand:
"The Moors of Shadow: This is the name of the vast and cold stretch of land in the center of the principality It is dotted with a few small woodlands or marshes. Most villages of moth are scattered throghout his land".
"The Inner Valleys: The vast western area of Moth near the Miurenheim range is a cluster of small mountains and isolated valleys Forests and villages spread throughout the inside of the valleys, some of which go decades without receiving visitors".
...And? What am I supposed to do with this info? Bland and useless. Lonely Planet is not what I paid for.
The second flaw is that the book is undetailed about the most relevant stuff.
Let's take Dalaborn as an example. It is supposed to be "an army with a state" as opposed to the normal "a state with an army". So why ain't I getting any details of how this army is structured, some flavour about it, some plot hooks about it, its customs and "subculture etc... But I take two pages of boring-ass description of geographical features? Same thing on, say, Phaion commercial guilds.
The other big flaw of the book is that it hints alot to hidden truths but not only it does not explain them completely, it doesn't say sh*t about them period.
I've already discussed the matter on these forums; some people think it is a great feature because "it lets the individual gms fill the blanks in for their campaign".
To which I reply: the blanks should not be bigger than the written parts, or we have a problem. Well, here we have a problem.
The orgainizations, which are great creatively, suffer of the same "not enough detail" problem. Magus Order and Yeudah are acceptable, given they should be minor organizations. The lack of detail on Tol rauko, Wissenschaft, Black Sun, Samael is not. Also, the info on Empire and Church, while a bit deeper, is too scattered.
Last but not least, almost no work done on the personalities and behaviours of the npcs, even important ones.
All in all I'd give the book a 6 out of 10. Some great ideas, not worked upon enough; a lot of info, too much of which uninteresting.