For primary children, its length will be intimidating. While some enjoy chunky reads, they are the minority. Even for Potter aficionados, a 640-page novel is daunting, especially when the hot action is confined to the final quarter.
Rita Skeeter and her Quick Quotes Quill provide a degree of comedy, but a fair level of sophistication is required to appreciate scenes such as Rita's interview with Harry. Here, the Quill composes snappy tabloid journalese from the oy's non-committal responses. As in most of Rita's scenes, the satire seems expressly placed for adults.
Harry's duel with Voldemort at the end of the novel is the darkest, most disturbing (and most powerful) writing in the series so far. If there had been doubts about Rowling's ability to write tough and dramatic narrative, this dispels them.
Voldemort is not just bad; he is truly evil. Gory scenes of self-mutilation will make readers squirm. It is supposed to be ugly and uncomfortable, but is it too strong for the middle primary years? Probably. Read volumes one to three first and leave this for Years 5 and 6.