I find it both puzzling and amusing at the same time that those people who accuse Kirino of being a boring "Mary Sue" actually hate her all the more.
Folks, that's the whole POINT behind her character. She's the classic overachiever who at the same time has serious issues with her interpersonal skills. Even the author herself said that she finds Kirino insufferable and the least likable out of all characters
The way I see it, Oreimo is a character study first and foremost. And from this perspective, I consider it exceptionally good. As someone who also had a little sister 3.5 years younger than me, I can attest that several details ring oh so very true to me (while others are obviously different).
If you can't enjoy this central aspect of the show, you probably won't be satisfied. But I'm perfectly okay with content and pace as-is. I'd just love to see a 2-cour version of it, to reach the latest book.
A solid 8/10 from me.