I think it's kind of brilliant. I aged twenty years watching it, but in a good way. My only objection is that I'd been hoping for a different ending, but interestingly the original author agrees with me. He wasn't allowed to write his planned ending for the original light novel series and so he's since released a short after-story called "A Momentary Chance Encounter", suggesting strongly that Kirino and Kyousuke have created a double life. Their relationship in public isn't their relationship in private, which includes sex.
Apart from one factor, this is a good but pretty standard romantic comedy. It even turns mildly harem-ish, with other girls' interest in Kyousuke complicating the situation more than a little. (Kyousuke and Kirino are both unstable cocktails of jealousy, possessiveness and self-sacrificing determination to be supportive.) And that's without considering the feelings of the other girls. My heart bled for Kuroneko. I love everyone in the cast, in fact. Season 1's tsunderes are still a handful, but we're getting a clearer look underneath their emotional armour. Admittedly I wouldn't have minded a stronger focus on Saori (who's probably the nicest girl in the cast and is basically a nerd ray of sunshine), while Manami's mostly relegated to a supporting character. However those aren't problems. This is a memorable story, well told, with a vivid cast and strong emotions.
This is a show with nuclear weapons armed and ready, at any point out of nowhere capable of making you want to curl up in agony and die. However it also has a lot of subtlety. Trying to read what's going on in its characters' heads can be fiendishly difficult, especially in the earlier episodes. How much does Kuroneko know/guess? Does Ayase really have a crush on Kirino? Why in the name of all that's holy did X do Y and how deep does their motivation go? The animation also includes some fantastic facial work, with Kuroneko's most expressive moments by far being non-verbal.
There's a continuity quirk. There were two versions of Season 1 ep.12: the TV broadcast one and the net-release "True Route" one. Season 2 is a sequel to the latter.
The show also has amazing DVD extras, which are fortunately on the English-language DVDs too. Most of them are Flash-animated bonus episodes. They look terrible, but each one's a good ten minutes long and might even occasionally go on too long. They're not canonical, whatever that means. They're just comedy skits, but they're often very funny. My favourite is "Charged Otome Road" (e.g. Kirino vs. Sena arguing from a very wrong point of view about whose brother is the biggest pervert) but there are also laughs to be had from a similar pairing of the brothers (i.e. Kyousuke vs. Kouhei).
The most lavish, though, is the Bonus Game Episode. Kirino loves playing adult video games, so this is a double-length episode animated in the style of a video game. (42 minutes long. This is thus effectively an 18-episode season.) It introduces yet another haremette, but Kirino has some eyebrow-raising reactions. Sakurai suggests something lewd. No dice. She gets shot down. However after a later suggestion of something even more outrageous, Kirino says "I feel like in the end I usually end up being forced to carry out the plan you always bring to us", followed by "this one I'm really actually not doing." Goodness me. That said, though, Kirino only does a super-innocent version of number three, so...
What's more, Season 1 had DVD commentaries from the fictional characters! They watch the episodes, passing comment on each other. Kirino even interviews her voice actress on what it was like playing her. That sounds brilliant and I wish so much that they'd been on the English-language Season 1 DVDs too.
Fundamentally, this show is a romance. It's funny, but you'll be watching mostly because you wish the happy couple all the best and you're hoping they get together in the end.
Despite the incest.
That's that nuclear weapon I mentioned. There are many reasons why I love this show, but there's nothing exceptional about most of them. The incest angle, though, is hard on the nerves, especially when being used for comedy purposes. There are things you shouldn't say in public. Then there are things you really, absolutely, definitely shouldn't say in public. Then there's this. Kyousuke's the worst for this, although he's not alone. He's a giant ham when the scene calls for it and... oh my word.
That's what makes this show (within my limited experience) unique.
Apart from anything else, since the start of Season 1, I've been wanting them to keep their voices down at home. If Kyousuke can hear Kirino through the wall, then who's to say that their parents can't too? Mind you, at least they dodge the paedophile angle I'd been worried about. Kirino was 14 at the start of Season 1, but she's 16 in the final episode and there's no concrete evidence that they ever did anything anyway. (Lots and lots of awareness of the possibilities, though.)
Their backstory (ep.13) fits too. You see how close they were before everything went wrong, in a way that's so natural and yet also really sad. (You could even argue that their years of mutual alienation undermined the Westermarck effect and hence helped to create a situation where things could turn out like this. That's not the half of it, though.)
Silly observation: "Oreimo" is short for "Ore no Imouto ga Konna ni Kawaii Wake ga Nai" ("There's No Way My Little Sister Can Be This Cute") but the abbreviation itself could mean "I am a potato".
There's nothing here I don't like, except to some extent the open ending and that's since been subverted by the novels' author. It just about killed me. I couldn't stop watching it. My skin would probably crawl right off my flesh if they ever did a live-action adaptation, though.