I think I've figured out my reaction to HP5 by analogy. With the first few books I
couldn't make a valid comparison between HP and Susan Cooper's The Dark is Rising.
I like the latter much better; the initial HP books were fluff in comparison. Now
after Books 4 and 5 the comparison is mch closer, as Harry Dmbledore, et alia face
issue on similar levels to those in TDiR.
And now that I can compare
the series, I'd say that this book is equivalent to The Grey King I don't
love that one the way I do The Dark is Rising and Silver on the Tree
but it's necessary to the series; it introduces important characters (a major
factor in TGK, less so in HP5), sets up situations, and matures the central
character to where he can tackle his ultimate battle.