Saw it tonight - absolutely A+! Up was brilliant - it was heartwarming and heart-tugging, both, and even without dialogue, "Kevin' was amazingly charming and sympathetic. I agree that Muntz could've even been eliminated without losing what made the story work, but as it is, he was a perfect wake-up call for Carl. As always, Pixar are at the top of the industry in technical perfection - the rendering was breathtaking at times. But it's the story where Pixar really shine, and I think they've definitely topped themselves once again.
One observation about the 3D: this is the first time I've seen a 3D movie since The Creature from the Black Lagon, and I was very impressed by how well it works. But somehow, even though the film was obviously widescreen, and looked it with the naked eye, with the 3D glasses on, it seemed more like a 4:3 format, and overall it felt very small and initimate, not wide and expansive as these films usually do. I'll have to see it in a non-3D theatre to get the full appreciation of what I've seen, but so far, I'm very, very impressed with Up.