I'd be surprised if many people actually like her. She was designed to he hated. She was manipulative and selfish, and then of course there was the whole homewrecker thing. If she hadn't been all of those things, then some problems would have been raised. If she'd been like Oz--likeable, normal when not a wolf, funny--then when Oz cheated, the viewers might have remembered that time Willow kissed Xander, and we might actually end up sympathizing with Oz for the way Willow reacted, because technically he can't control his actions at the full moon.
So yep, she sucked, but she had to.