The movie followed the original book very well and only diverted in a few, mostly insignificant places. There were even a few nods to other Dr. Seuss stories in the background. But at the same time, it's clear that Jim Carey had a significant amount of creative license with Horton's character. There were quite a few scenes where his traditional humor style showed through and also a number of scenes that were more stupid than funny. When are major movie studios going to learn that mock Anime (aka Japanamation) scenes just aren't funny anymore?
But aside from the Jim-Carey-Stupid-Scenes, the movie really was pretty good. I'm a big fan of Dr. Seuss books and cartoons and Horton Hears a Who certainly maintained that. The characters all looked like they'd stepped off the page of a book, and yet also looked like the actors playing them. Even the Kangaroo actually looked like Carol Burnett. :-)
So all in all, I enjoyed the movie. I'd give it 3 stars out of 5.