I can't tell you why many people dislike him. I can only tell you why I, personally, disliked him. I didn't like his attitude, all this "Uh, I'm the hero of this whole game blah blah blah". (I also disliked Balthier in XII for the same reason xP)
I agree that he is true to Serah - but apart from that, he was just annoying. He was talking way too much, always ran headlessly into danger, always pretty aggressive...

As for XIII-2: I was pretty happy he didn't appear that often. Still, I wondered how he could be okay with Serah wandering through time and space. If you really love someone, wouldn't you go with her? Or would you not at least follow her?
(Have you played XIII-2 as well? I'd like to read your opinion on Snow in that game as well ^^)