First off, this is one of the most visually stunning BDs I have come across so far. A delight to look at from start to finish. The story, which at first I thought sounded quite stupid, turns out to be simple enough for young children to understand (my 3 year old loves it) without being boring for adults (at least for the first 10 viewings, kids love to see things over and over). The ending, as the other reviewer hints at, is truly uplifting, with the joint effort of the Whos to make themselves heard. The humour is usually inclusive for adults and kids, only rarely too obscure for kids (Kissinger impressions, JFK parodies etc), only rarely too goofy for grown-ups (Horton swallowing a leaf-bug at the start seemed pretty expendable to me). Steve Carrell is brilliant throughout, and Jim Carrey shows for once that he does not have to be mind-numbingly annoying to get laughs.
There are good extras too: an Ice Age short and a free digital copy (albeit only good for use on one specific device) and entertaining making-of features etc. Perhaps the only gripe might be that at times there seems to be some more overt than covert political recruitment of young minds going on: the Mayor at times come across like an environmentalist faced with the occasional public reaction of "Oh, well, global warming means better weather dunnit?" (the chairman's kite-flying quip) And is "a person's a person no matter how small" an attempt to sneak some Pro-Life in there too? None of this spoils the film, but I don't care for indoctrinating children, however worthy the cause. Maybe this is oversensitivity on my part. I still loved the film and would recommend it wholeheartedly.