My problem with the Dummies guides is that they add too much 'fluff'. I get impatient sometimes reading them, because it frequently seems that they could convey the same information in half the text. But the information they do provide is good, so I have to admit that I have a number of Dummies guides on my bookshelf. Not sure I need one for D&D, but it would be interesting to see how the information is presented.
Now if Peachpit would do a Visual Guide to D&D, that would be interesting to see. Probably not useful, but interesting!