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 before the finale if you insist on being a completionist.