Before I get to the actual review, I wanted to say in advance that I never had the opportunity to read Dr. Seuss' books, most likely thanks to living in the middle of nowhere, aka somewhere in Europe
I think it's a good movie. It gets its message across, it does manage to create that whimsical feeling that I assume is present (and probably even stronger) in the book, it's memorable, and has just the right amount of funny and heartwarming scenes to work. The animation is good and manages to be remarkably slapsticky and cartoonish despite being made in CGI. The designs of characters are good (maybe aside of the monkeys, whom I found to be a bit creepy
) - I especially liked the design of the vulture named Vlad Vladikoff, who had a likable yet sufficiently creepy look to him.
The only problem I have with this movie is that while it seems to aim for being a harmless movie perfect for even the smallest kids and still enjoyable for adults, it comes off as surprisingly mean-spirited at times. A few examples being the council the Mayor has to deal with and their really mean attitude towards him (even going as far as insulting him), the jungle as a whole seemingly hating Horton, some of the slapstick being a bit too brutal, and ESPECIALLY the climax. It just felt out of place, as if the movie couldn't decide if it wants to be a gentle kid-friendly movie, or something aimed for a bit older audience.
Overall, the movie is definitely good, maybe with a few small bumps along the way.