I would say that the quest is not a bad book, but it is so far removed from the others as to almost alienate large sections of his fanbase. I have read all of Wilbur's books and this one stands alone as a fantasy novel rather than the well researched historical adventures that i am used to.
If you are into fantasy then the book is quite a good read; Smith describes fantasic and colourful scenery and characters.
The problem is that almost all of Smith's fans aren't. If you are used to the realism of his great books like When the Lion Feeds and Birds of Prey then you will barely believe what you are hearing. To an average Smith fan, the plot is outlandish and at times laughable. It's sad to see that so many people hate this book, because it really isn't all that bad, but Taita is one of Smith's most loved characters, and perhaps his first dive into fantasy shouldn't have used him as the Guinea Pig.
There are also quite a few consistancy problems with the earlier novels in the series which serve to annoy long term fans. (When did Wilbur decide that Lostris' baby name was Fenn?!)
Wilbur Smith has set a very high standard for his novels, and this one does indeed fall short of it, however i get the feeling that if this was his first book there would be lots of fantasy fans giving it four stars rather than the miserable ones and twos it's getting. He's done a very brave thing jumping into the genre of fantasy, but it looks like it's not going to pay off for him unfortunatly.
Also it's really, really, really long! It could be a series in itself - Perhaps that would have worked better; a fantasy series about the fight against the lie...
Nice try Wilbur, but lets have a good old African adventure in 2009 before you put all your fans off for good. I'd like to see what happpens to the Courney family after Blue Horizon!