All reviewers of this movie seem to either love it or hate it, and it's easy to see why.
As has been pointed out to death, the "modernized" Hollywood story added in of a celebrity-voiced kid trying to win a girl and overcome a two- dimensional villain in the process is thoroughly uninteresting, and will make older moviegoers angry at the lack of effort made in expanding the story.
The Once-ler's tale, on the other hand, is the story carried over from the book, one of unchecked ambition and carelessness. The Once-ler is morally gray and this is done very well, as he is likable, yet you're never sure- is he a villain? A misguided hero? Somewhere in between? Sure, it's padded out with cutesy animals, but knowing what becomes of them in the end makes it considerably less innocent. This story is where Seuss's message is, and it still makes it through.
The score by John Powell is epic and the animation and designs are gorgeous, which add great atmosphere in the darker parts of the movie. The environmental message is very un-subtle, and people who dislike that in other movies will dislike it here.
One need only listen to the cut song "Biggering" to see how powerful this story could have gotten. And they just didn't have the guts. But beauty does manage to seep through in places, and this is one movie that I'd highly recommend seeing only parts of.