I loved this movie so much when first seeing it that I had pre-ordered the DVD when it first came out (something I don't usually do). I've watched it countless times and haven't gotten sick of it. Now, seeing it in high def, the movie looks and sounds absolutely spectacular.
Since you likely know the story, I'll only comment briefly on that. This movie dares to tackle some very important issues, some for children, but some that even parents could benefit from. The idea of accepting someone because they're different or believing that you can still do anything despite physical handicaps, is a message that everyone should listen to. Looking over all of the adventures that Nemo has, he really can do anything that he sets his mind to. Even more profound, though, is the idea of over-parenting. And we hear it summed up so well by Dory, the most unlikely character to deliver such a message, when she says, "Well, you can't never let anything happen to him. Then nothing would ever happen to him." As much as we'd like to keep our kids confined so they're safe, we also have to learn to let go. Notice how at the end, when Marlin does let him go to school, Nemo swims back to say, "I love you, dad."
Now on to the blu ray. Back when this movie first came out, blu ray wasn't even a trademark term. So we looked at the DVD and figured the quality was good enough. Seeing this beautiful film on an HDTV is simply amazing. The magnificent colors, the tiny details of the fish and the ocean, and the 7.1 surround sound is just beautiful. We sometimes get disappointed with blu ray conversions when studios seem to make little to no effort to improve the sound and picture quality. Not the case with this one. I just can't stress enough how perfect this movie is going to look on your big screen.
I'd consider this to be one of Pixar's greatest works, from animation quality, to story, to casting. On top of all that, the improvement to high def is well worth the price. This is one movie no family should be without.