Picked this up at a library book sale. I read a lot of Roald Dahl as a kid, but never this one; and now I can say I don't think I missed much. The plot is pretty thin and lacks that zaniness that Dahl brings to the books I did like as a kid (The BFG, Matilda, The Witches, Charlie and the Chocolate Factory, James and the Giant Peach) -- I really didn't care whether Mr. Fox got shot by the farmers or not. Meh.
I'll probably sell or give this one away before Z's of an age to read it. Things to watch out for: gross farmers, a drunken rat, characters telling each other to shut up, glorification of theft (though that's hardly unique to this book).
Age I'd let Z read it, if she really wanted to: 7 or 8.