The appeal of Half-Blood Prince is how Rowling makes these things somewhat ambiguous. In the second chapter, I was shocked at Snape's admissions. Rowling has done great things with that character through the audience. She spends most of the first book trying to convince you he's evil, and then the next four books trying to convince you that he's a complete ass but, at his core, he has his priorities in the right place. Plus, like the other characters in the book (besides Harry), you can't help but believe that Dumbledore has some fail-safe logic to trust Snape. So I did my best to try and convince myself that Dumbledore was right and that Snape was just playing as Voldemort's follower even though the evidence continued to mount to the contrary. I had no doubt that Dumbledore would not survive this book, but I was surprised that it was Snape who cast the finishing blow. I almost expected it to be a joke at the end, some kind of test or trick.
I'm going to read the book again, but, now that I've had a chance to step back and think about it, I don't know if I believe the hype that Half-Blood Prince is the best book. As Part 6 in a 7 part series, it does very well. But as an individual book, it seems to lack an element that ties everything together. You would think it's the Half-Blood Prince element since that's the title of the book, but, if you really think about it, what was really the significance of the whole Half-Blood Prince storyline? It gives us a little more insight into Snape's past, but not nearly as much as Chapter 2 does by itself. The background on Tom Riddle, though, was done very well, and I extremely enjoyed it. However, it isn't until the last 100 pages or so that the plot really starts to come together. Up until then, it just seems to be floating in between the Half-Blood Prince, chasing after Malfoy, and who's dating whom. It's a lot like Philosopher's Stone in that it takes a while for the real plot to get off the ground. I did like how they switched things up a bit by having Snape be the new Defense Against the Dark Arts teacher. The special guest star teacher of the year was getting so cliched that it took me completely by surprise, and I loved it.
But, like I said, it is great as Part 6 of 7. You learn a lot more about Voldemort/Tom Riddle, horcruxes. The relationships between the characters continue to develop, and Dumbledore's death has set the stage for Harry to take matters into his own hands as a hero (which is why I knew Dumbledore would die).
I'm not sure why you put the "especially" after Hermione, ADM. Unless I've forgotten, I don't see any reason why Hermione would drop out as opposed to Harry and Ron. Harry has the only strong reason. But I think he will be at the school for at least some amount of time simply because "Year 7" wouldn't make much sense if he was not in his seventh year at Hogwarts. Plus, I'm dying to see a graduation ceremony. It's bugged me forever wondering what they do to celebrating graduating students. I've had people suggest the Regulus Black theory to me already, but Borgin or Burke is an interesting theory too.
And I agree with your assessment of Harry Potter haters. I should know because I was one once. I let the "too cool for it" opinion as well as parodies shape my opinion of what Harry Potter was like, and, from that perspective, it seemed really stupid. A dorky-looking kid with a lightning-shaped scar does magic tricks and gets himself in all kinds of random little adventures. It didn't sound like very compelling reading. I'm glad I got my nose out of the air and decided to try it.