One of the reasons for the hate up here in the Great White North is because they're Canadian. We have CRTC (Canadian Radio and Telecommunications Commission) regulations that stipulate a certain amount of Canadian content per hour on the radio, which basically means you're going to be hearing a lot of Nickelback and RUSH, and, as they say, familiarity breeds contempt. (Except for RUSH. RUSH is awesome.)
As far as them being sellouts, well that's really not much of an argument. Nickelback has always said that they're in it to make money and get chicks. You can only be a sellout if you started with some pretension about it "being just about expressing ourselves" and then decided that the money was more important than the music. Nickelback went into things with the attitude that it was about the money, and they happened to make it big. That's another reason for the hate; the pretentious indie fans can't abide the fact that a band that has never released a protest song or railed against the government or the music label system can be popular. Or, they're the most pretentious of all, the person who automatically hates everything that's successful. (Such a person really hates it when you point out that their taste in music is driven by exactly the same thing as all the teeny-bop pop fans. Had a friend who thought Evanescence was one of the greatest bands ever ... until "Fallen" started to do well in stores, at which point they magically became terrible.)
As far as I'm concerned, they're ... ok. They got some good tunes, they've got some that are less good. Let me put it this way: they rarely make it onto my mp3 playlist, but if I hear them on the radio I don't change the station.