I have an interesting perspective because Advent Children was my introduction to FF7. I liked it when I first watched it even though the plot was completely incomprehensible. I liked the action scenes, and at the time, the animation was superb.
I started to like it less after I played FF7, and much of it has to do with the original game's ending. FF7's ending is ambiguous. Near the end, you're told that the Planet could very well sacrifice humanity in an attempt to save itself from the Meteor if it felt as though humans were also a threat. And when you beat Sephiroth and the Meteor finally touches down, the Planet summons the Lifestream to destroy Meteor, but you ultimately never see what happens to humanity. The most you see is the ruins of Midgar centuries later. Sure, you can hear the laughter of children, but my takeaway is that you ultimately don't know if the Planet decided humanity was a threat too. All we know for sure is that Red XIII's species has survived and Midgar was destroyed. It's a bold way to end a game in my opinion, and I love that it's all shrouded in ambiguity.
Until you watch AC and find out, oh everything's fine. The humans? They all moved to a new city. To me that removes a lot of the mystique of the original ending, and in a way it devalues what that ending was going for. And this is a problem with a lot of the Compilation; it either adds nothing to the original story or it actively detracts from it.
There are other reasons too, like Cloud becoming a whiny emo bitch, forever coloring modern perspective of him, but for me personally it's the fact that the movie tries to provide an answer to a question that was never meant to have one, that being the original's ending.
