Fans know the tales better than I; I stopped reading the books after the third entry, and have only enjoyed the movies intermittently. The first two, directed by Chris Columbus, were too faithful to their source material to have much life or personality. The next couple had moments of macabre delight in part because they strayed from their predecessors' literalness but also felt curiously static; formula crushed the energy from them to the point that I skipped the fifth altogether. I did see the sixth, but cannot remember a single frame.
I'm not saying you have to be a fan to review the film, but it's the final chapter of a larger tale. If you're going to grapple with it, then you really should have some working knowledge of what has gone before. This kind of thing is like reviewing, oh, A Tale of Two Cities and saying, "Well, I didn't really like the first ten chapters, skipped a bunch of the next ones, but here's my take on the book's last six chapters anyway."
Ultimately this goes back to my deep, deep aversion to the whole notion of "objectivity" where Art is concerned. I remember arguments I'd have on rec.music.movies, for instance, where someone would say, "Well, the score to [Movie Title Here] is nice, but when you listen to it objectively, you realize it's not that good." I'd respond with the obvious point that no, you're not being "objective", you're just being negative but trying to give yourself an air of authority by claiming "objectivity".
I submit that if someone says "I really don't know anything about Harry Potter", then their opinion on the new movie is next to useless. And that goes for everyone, including Roger Ebert. If someone who loves Harry Potter doesn't like the film, that's interesting to me. Otherwise, it's just people who don't like spinach offering reviews of a cookbook of spinach recipes.