Go see it. I don't want to give anything away or build up this movie so much that it's a letdown compared to how I make it sound. Just know that it will affect you in ways you didn't realize from the previews, and that if you don't absolutely love it, you have problems. ;)
With every new Pixar movie I see, I think, "Wow, they've really outdone themselves this time." Everything they make is so clever and has at least SOMEthing that is relevant to people of all ages. (I wasn't a huge WALL-E fan, but that was one of those that people built up to be the absolute SQUIRREL!...best movie I would ever see, and when it wasn't that, it seemed "ehh" to me, which is unfortunate.)
"Toy Story" and "Monsters, Inc." are two of my favorite movies, but "Up" topped them. I went expecting a simply cute and funny movie and got the surprise of finding my new very favorite Pixar film. (For the record, it offers "cute" and "funny" in spades, but also so much more.)
Just go see it. Please!