It seems that there are two reasons for the hostility towards Evans. Firstly people genuinely don't rate him, secondly he's ex United.
So, what if he turns out to be a decent signing? If he exceeds our expectations?
Players that turn out to be better than expected usually are extremely popular with fans, not just at City but everywhere. We liked to be proved wrong, love it when a signing turns out to be a jewel (even if only semi precious) rather than a turd.
A few posters have mentioned the spitting incident and him being a United fan. But I don't really think the animosity towards Evans is personal. He's not a Keane or Neville. If we held a poll of the most hated United players he wouldn't make the top 50, probably not the top 100. The real reason for the animosity is that he's one of their cast offs, and we thought the days of City signing United cast offs were long gone. He's not a Tevez, a player their fans really wanted to keep. He's a player that they sold cheap to WBA. Which means that United fans will be taking the piss out of City fans because we've signed a United reject. And if he flops, costs us the league, they'll keep on taking the piss. The flip side though is that, if he succeeds, he provides plenty of ammunition for City fans to take the piss out of United fans. We'll dine out on one of their rejects lifting the PL or CL trophy at City.
I can see him becoming a cult hero!