I consider the Harry Potter series as sort of a DaVinci Code with a broader target audience. They’re both entertaining and fast; so while they’re not likely to change the reader’s life, they are enjoyable, unlike the books most are forced to read in school.
Another positive to the Harry Potter series is that they seem to get better with each installment. One of my complaints about each book is the time devoted to getting Harry to school. This one (#4) is quite brief in getting Harry away from his ‘muggle’ relatives and into the wizarding world. The mystery is fairly sophisticated, with smaller mysteries along the way, and it does a good job building on the previous episode (The Prisoner of Azkaban).