Horton Hears a Who is a difficult film to review because it doesn't do anything wrong, but it doesn't do anything particularly good either.
It's an average film, with average characters, an average narrative, and average animation.
I didn't connect to anything in the film but I was never bored and it would appeal to children, so I can get my score slightly positive because of this.
I did like the message of "a person's a person, no matter how small", and this is definitely something that children should be taught. If it's in the form of an animated film, then so be it.
Overall, I had a decent time watching it and the ending does get a little more exciting, but it's not a film that I'll be recommending to anyone without young children.