I was kind of cynical when I bought these; I figured JK was trying to cash in on her success. But at least she's cashing in for charity, and I was desperate for *something* HP, so I bought it.
The books, however, are totally awesome. Like Tolkien's "Silmarillion" on a much smaller and more whimsical scale, they provide glimpses into the larger imagined world in which HP is embedded. They do not merely parrot what we know from the books; there are tons of fun nuggets of information and mysterious hints.
"Fantastic Beasts" gives us the 411 on our familiar friends the dragon, the acromantula and the flobberworm, as well as tons of new critters like the Lethifold. The margins have hilarious "comments" by Harry and Ron, slyly referencing the published HP novels. This is a must-have reference for fanfic writers in need of story ideas.
"Quidditch Through the Ages" is a very cool look at the history of Quidditch, full of Rowling's usual lighthearted social satire (i.e. Americans neglect Quidditch in favor of their homegrown broom sports :) This book in particular gives us the feeling of having been ripped from a larger unwritten history of Harry's world.
As far as I'm concerned, once she's done with books 6 and 7, Rowling can keep writing these nuggets for the rest of her life :) I would love to see her write "Hogwarts, A History" or "A History of Magic" by Bathilda Bagshot.