I doubt they'd have any serious disagreements. It's more that How to Brew was written a decade ago and is well out of date in its treatment of alkalinity in the mash.
Well, no, because myself and others have raised those concerns with Mr. Palmer within the last year, and he still seems to think his spreadsheet is a valuable tool for homebrewers. On that point, I have voiced my immense disagreement. Here was a thread from a while ago, for anyone that missed it:http://www.homebrewersassociation.org/forum/index.php?topic=11944.0
The long and short of it is, I'll wait for a detailed review of the topics covered before I'll consider plopping down money for a copy.
Edit: I want to make clear, my main problem with Mr. Palmer's approach to water is he never actually said how/why he came up with the RA numbers he recommends. I asked him directly if it was based on his own empirical evidence, professional brewing literature, or some other source, and he ignored my question. To me, that sounds like it's just his WAG, and I have a very hard time taking him seriously on that topic, or any other. YMMV.