IMO, the 3rd and 4th books are really good children's fiction*
the way she draws in so many social and political ideas, and deals with them in an accessible but mature manner, is really quite admirable.
when i have kids, i'm quite sure i'll force them to read HP (or, more realistically, i'll simply nudge them in the direction of Potter, and let the inevitable addiction take hold)
* i haven't read the 1st/2nd. i saw the films on DVD, because at first i, like many of you, was resisting being drawn into the HP fad.