It's interesting, I pretty much loathed OotP and HBP on first read. In OotP, I felt Harry was simply too whiny and when reading HBP for the first time, I also found the pensieve stuff too "flashback episode" as well.
On subsequent read-throughs, though, in context of the entire series it's totally understandable that Harry is so angry in OotP. He's been handed a **** hand of cards in life and he's kind of just realised it. I would be just as angry!
Also, I loved that insight into Tom Riddle once I knew the ending. I'm not sure why but it just feels like a puzzle piece. Knowing what happens with the cup and the locket and how it all plays out. Now when I read back those "flashbacks" are fascinating to me whereas the first time I could barely get through it.
I had a similar experience with Stephanie Meyer's "The Host". It was an unbearably difficult book to get through the first time. I tried several times and ultimately had to listen to it on audiobook. Since that first read, though, I've read it numerous times and love it and actually think it's quite genius the way it's written. Though I can't tell if that means it terrible writing since it was so hard to read the first time or wonderful writing since the story is so clever!