Hey Wolf, Rys actually news posted this thread on the frontpage in hope of getting some extra exposure - not sure how much of an effect that'll have, but we figured it'd be worth a shot.
Overall, I've personally only got good things to say about ShaderX5, and the ShaderX Series in general (I got 3, 4 and 5). It's especially nice that you're sticking to a regular schedule for them, so that it represents the cutting-edge more than GPU Gems or articles in other game programming books. It would be nice if the delay from idea-to-publishing was shorter, but that's obviously not really up to you!
One thing I'll admit not to like too much is the organization fo the book. I'm not asking for a GPU Gems-like coloured frontpage with numbers - it's nice, but arguably is overkill. The problem for me is more how the different chapters are organized in a single topic, and also their naming scheme. It's fundamentally impossible to guess if a chapter represents an idea that's usable for a game, or only cinematics, or even only large-budget films given how slow it'd be to render.
One "easy" fix for that might be to order the chapters in one category based on how "fast" the technique is likely to be and usable it'd be in real games coming in the 2008-2009 timeframe. Or potentially just rename some chapters to clearly indicate it's "For Games" or "For High-Quality Interactive Rendering". Things like that. Anyway, this entire comment might be arguably just be a small thing, but it's still the biggest thing I could think of, heh!
As for content, one very normal trend in recent ShaderX books is that chapters are more and more specialized. This is both a good thing and a bad thing; as 3D programming further extends and perhaps complicates, specialization is to be expected. However, I know I'm personally still a big fan of chapters proposing a solution to a specific problem (water; terrain; sky; particles; etc.) rather than a really new technique.
I don't know how other people feel about this, but I do really like well done chapters that deliver a good summary of the state of the art in a specific domain and an incremental improvement to go with it. Of course, this depends completely on people actually proposing you to write that kind of chapter - but if that was the case, I think it might be worth it if you are confident in that person's ability to write on the subject.