While I am of the opinion that the Harry Potter series hits a peak with the previous book, that doesn't mean that the remaining books are not excellent. They are engaging and draw the reader along relentlessly (with one exception that I will mention in a later post), full of invention and plot twists.
However the size of the books grows alarmingly, starting with one, and I don't believe that just because an author has a head full of ideas and a complicated plot that their publisher should indulge them. I'm sure any of the last 4 books in this series could be improved by a ruthless editor. This is something of a modern trend, and I don't blame publishers who need to make a quid in a tough market, but the quality of a bloated book suffers from this lack of discipline.
Minor quibble. Enjoy, take your time and go along for the ride.