What does "value" mean to you?
If you mean, "Can I use them as reference books when I'm playing D&D at the table?" the answer is no, because that was never their intent in the first place.
They're extensive first looks, first and foremost, but the essays written for these books are not going to be regurgitated in later products...so there might be world-building details that don't appear elsewhere, or only in shorter form. I can already think of a few mechanical details that have already changed from when I turned it over to typesetting, which will provide further insight into the process. Certainly the designer commentary is unique.
Value is relative. I bought the Absolute (super-deluxe) edition of Darwin Cooke's New Frontier comic book not because I expected more of the actual comic, but because for me, uber-comic geek that I am, the sketches and commentary and details about the process of making the comic are just as much a part of the reading experience. Would I be missing anything from the story without them? Maybe not. But those things add a huge amount of value to me.
Michele Carter, RPG Senior Editor, Wizards of the Coast