To answer your question - all the Cho hating began after the fifth book. All of Harrys' fans are up in arms because she had the audacity to (gasp) go out with him and (the horror) not like him too much afterward!

I don't hate her at all, and I actually find it a bit annoying that people are bashing her. She just lost her boyfriend, she's trying to move on and she probably thought, Hey... I liked Harry a little before Cedric, why not give it a go with him? Things didn't work out and that's that. Relationships don't always work out, especially first ones and Harry will find someone else. It wasn't her intention to use or mess with him.

She's not overly emotional, as a lot of people are saying. What she's been through is really scaring, and I think she has every reason to cry. If she's still doing it in five years - than it's like, yeah... time to get over it - but for now, it's been less than a year and I think she has a legit reason to be sad.