Man, watching Sisters Arc from Railgun S is as creepy as **** since they placed so much emphasis on the Misaka clones. It wasn't even that creepy when I watched the Index version :S
Yeah, having backstory for Sisters' Arc is awesome. And yeah, Angel Fall was not as good since Index/everyone is just derping around. It's cool to see Touma's parents though.
It looks interesting, since Nagi is the first romance-mailny anime I've watched.