You've got to hand it to Disney. In a lot of ways, I'm in the same buisness they are. We both sell fun. The Disneyland fun machine is a work of art (literally, in a lot of ways.) I know it's not the same for everyone, but as soon as I walk through the tunnel onto Main Street USA, I've got a huge smile on my face. Part of it is nostalgia. I first went to Disneyland in 1976, for the bicentennial celebration. I was 6. The sights, the sounds, and particularly (this may sound weird) the smells of Disneyland bring me right back to the best parts of my childhood. I don't know if its the same for folks who go for the first time as adults, but from the smiles, I'd say it's still pretty good. If you have a 5-10 year old, and you can afford to go to Disneyland or Disneyworld, GO. They'll love it now, and they'll love it again in 20-30 years when they take their children.
If you went to Disneyland as a kid like I did, and want to relive some of those memories check out my favorite Disney fan site, Yesterland.