It starts well, I reckon, and ends very well. But in many ways it just doesn't match the others.
I liked the tone to the first chapter. It was different, amusing, introduces Scrimgeour well and gets Rowling's point across about Voldemort being so bad even the Muggles know it. Chapter 2 sets the book's plot in motion and shocks you both when you see Snape and when he agrees to the Unbreakable Vow. I had a feeling from the start Draco's job was to kill Dumbledore; once Snape made the vow I knew Dumbledore was a goner. It was a fantastic start.
From then though, the book slows, and the pacing feels very uneven. The books have always used the school structure to carry things along and mix up a variety of plot threads, but this time it felt off, and the plot strands not spread evenly. The Half-Blood Prince is a major player, always on Harry's mind during the beginning of his time at Hogwarts... then is barely mentioned for ages before resurfacing. Their hormones suddenly rage and Harry lusts after Ginny... then again it disappears for a while before he gets the girl.
There just isn't enough to carry the middle section of the book IMO other than the general life around Hogwarts -- lessons, friendships, Quidditch. You don't feel a strong sense of urgency to the mystery at hand as with the first four books, and there's nothing inherently wrong enough with the world that needs fixing (as with Umbridge in the fifth book). The only hint you get of Voldemort's reign of terror is a throwaway line at breakfast where they casually discuss whether anyone they know has been killed. That's the extent of it -- even when one of their friends is affected, you don't see it having any effect on life at school. The mystery of Draco's plot isn't much of one compared to the rest of the series. It boils down to Harry wailing HE'S UP TO SOMETHING I KNOW IT and everyone else shrugging their shoulders and saying either Harry's paranoid, Harry doesn't have proof, or it's all under control.
The Pensieve flashbacks fill the void, but not well enough. They don't really build to anything and are just colour to fill the background. A couple of these could easily have been cut. What you really miss are the lack of memorable new characters. Umbridge was a royal bitch, but that worked well. Slughorn wasn't -- in fact, no more than an hour after finishing the book I actually had to go look up his name again because I thought it was Slugworth. After a promising start Scrimgeour rather evaporated into stereotype. McLaggen was the best hope, but he disappears from the last half of the book.
On the other hand, the book then gathers steam for the ending. Once Harry discovers Snape was the spy listening to Trelawney, it kicks into high gear for the last hundred pages or so. Dumbledore's death particularly, when Snape gets up there and you actually get to see him show which side he's really on. I knew it was coming once he made the Unbreakable Vow -- but you're still a bit stunned that he did it. It's not just that Dumbledore died, but that Dumbledore was WRONG. The revelation that Snape was the Half-Blood Prince (which you could see coming, because who the hell else was that good at Potions?) also hit home, because it then put clearly into focus that this book was really all about Snape. Which it was.
It seems there is quite a lot left to do in Book Seven. Aside from offing Voldemort, the fates of Snape, Malfoy and Wormtail have to be wrapped up, presumably not in one climatic battle. It'll be even odder with Harry away from Hogwarts, supposedly travelling with Ron and Hermione to hunt down the Horcruxes... though I have a feeling he'll spend a lot of time there. Remember, most of the Horcruxes are artifacts of Hogwarts' founders -- where better to start looking than at the school? Still, the school year format has served the series well in general. It's kept the pace going and gives a good chronological sense to events. But she effectively abandoned it towards the end here (when exactly does Dumbledore's funeral take place?), so it'll be really odd without it in the last book.
This does mean Harry won't take his NEWTs, but what good are qualifications to a dead wizard? I now fully believe Harry will die at the end of the last book. His brief relationship with Ginny (who has developed into a fun enough companion character) confirmed this for me. I never saw how Harry would ever hook up with anyone but Ginny -- she was written for the sole purpose right from the start. My initial feeling was that they'd end up together, but now that they had their fling, Rowling's ticked off another box on the Things Harry Must Do Before He Dies box IMO.
And a final bit of speculation: Who is R.A.B.? A certain relative was mentioned again, apparently in passing, and the means of his death explained again. Rowling rarely slips stuff like this in by accident. Could the finder of Voldemort's Horcrux be Regulus Black? He was killed by Voldemort, after all...
Overall... I have to say this is probably my least favourite book in the series. It's early days and I'll re-read it in a few months before I can properly place it, but there aren't enough good moments or new characters to lift this above any of the others. Mind you, the series has changed so much that it's odd comparing (say) Book 6 to Book 2. And I feel that this book will look and feel much better once Book 7 is out, because this is really a giant set-up, revealing all the pieces before everything plays out in the end.
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Do you think I'll be let down by the fourth one? Does it have the same pace/feel of the third one?