Literature Series 1 is good; Series 2 sputters a bit, then crashes spectacularly
When I saw Oreimo in Fall 2010, I thought that it was a wonderful treat to come out of that season. For once the somewhat pervy protagonist was female. The show had an intelligent sense of metahumor and self-aware parody that poked fun at the eroge fandom, other subcultures and anime tropes (especially those of incest-exploitation anime). It looked beautiful. All the characters were pretty cool as well, mostly communicating their own interests in an engaging way. Each episode also has different OP animation, and completely unique ED; I thought that was a cool trick.
And then came series 2. The first couple of episodes tried to capture the magic formula of the first series, but they didn't really seem to hit their stride and were just okay. Overtime, the metahumor began to give way to melodrama, culminating in an extremely disappointing finish. I can't explain at all what posessed the creators to abandon the parody, and then embrace the conventions being parodied.
I can definitely continue to ardently recommend series 1. And overall, I wouldn't call series 2 a complete avoid, but just don't expect anything earthshaking from it, and bail at the TV version of the ending if you insist on being a completionist.
Literature Kuroneko manga: a good spin-off
(The original title is Ore no Kouhai ga konna ni kawaii wake ga nai but Dark Horse calls their translation Kuroneko to make the shortened form recognisable.)
In essence licensed shipping fanfiction, I was drawn to this manga as I liked Kuroneko and felt the animated ending to be [redacted for readers of a sensitive disposition; ed]. It is not only better than I had feared, but also quite different from other such spin-offs (for Haruhi Suzumiya and Haganai). It is not a soulless cash-in, whilst avoiding the most common pitfalls of fan-with-ideas works.
Rather than shipping her together with Kyousuke, it is more focused at first on Kuroneko's own efforts to fit in with her new school, publish her writings in some way, and her rivalry with the big breasts of the big-breasted class representative with the big breasts (the artist could be subtler with the angles). Only from volume 3 is there anything resembling harem shenanigans, and I found them endearing rather than obnoxious. The two of them have about equal panels of blushing at each other.
Kyousuke benefits from being freed of his protagonist-mandated blandness; he is wiser, calmer, as much a mentor as a love interest. Kuroneko shows different sides, at times shy, conceited, hesitant, defiant, seductive, always falling back on her chuunibyou pronouncements. And unlike Haganai Plus, there is no bashing the love rival, which is welcome even though I would not mind seeing Kirino [redacted for readers with visual imaginations - ed].
I should mention that the plot is disjointed, at least for the first two volumes, and has plenty of contrivances. The art is generic, the same style as the anime but monochrome and less rigorous, though the panelling does a good job of livening up what would otherwise be talking heads. However the worst part is the out-of-place fanservice shots of the girls popping up like clockwork, it doesn't fit the romantic tone. Even the younger sisters are put in rather skimpy clothes.
If you want more of the better parts of the show, I can recommend this.