Sisters aren't horrible, they just don't have a real codex. IG is the codex that probably needs the fewest updates regarding Flyers next to Necrons (unless you want to nerf their Flyers, which is something they both need), and benefited greatly from 6th's changes. It's a good thing not many people play truly nasty IG, because they can be extremely ugly. Orks are solid, especially now that Nob Bikerz are T5. Nidz aren't bad, but they're not great. Unless they roll up good psychic powers. Nidz and Daemons are both stupidly good if you get the right combo of psychic powers and roll well for them.