So the plot of Half-Blood Prince is "Harry cheats at Potions and Dumbledore infodumps".
That, bar some snogging and an extremely drawn-out and overwrought ending, is it. That is all Rowling thinks is necessary to include in a 600-page book.
I actually quite enjoyed the first half: I think for me the heart of the series' appeal is the saturation of its worldbuilding, the sheer amount of day-to-day detail Rowling manages to pack in about the wizarding world, and Hogwarts in particular. So when Serious Horcrux Stuff starts happening, well, for me it's just filler to be skimmed through; which means that I found the second half much more tedious.
Harry is also beginning seriously to annoy me. I hate his obsessing over Malfoy, the idea that he considers himself so much cleverer than teachers who are, what, half a century older than him. Ugh, ugh, ugh. Just shut up.