I have always loved the idea of the Nintendo DS StreetPass. It is such a cool concept with a ton of potential. But I live in a very rural area so I almost never get a StreetPass. So when I was packing for my first trip to Disney World I just had to bring my DS with me!
StreetPass in the Animal Kingdom
So far I am kind of disappointed with the results, After spending all day in the Animal Kingdom around thousands of people I only got 3 passes. But maybe it was my fault. I am not sure how strong the StreetPass wireless is so maybe I was blocking off some of the signals because I had my Nintendo DS inside its case at the bottom of my backpack.

StreetPass in the Magic Kingdom
The next day when we were at the Magic Kingdom I made some adjustments. This time I did not have my DS buried deep in my backpack and I I got 7 StreetPasses. A lot better but still disappointing considering I was expecting at least 50. I am not sure if it was the adjustments or if I just happen to pass more people with a DS.

Worth it
Overall I would say it is worth taking a DS. If nothing else it gave me something to do when waiting in all the long lines. On two occasions we waited in a line for over an hour to do something. One was just to have a picture what Mickey Mouse because my 3 year old really wanted to see him. So I made use of my DS in that line.
I also could have just been unlucky with the SteetPassing or maybe it was just a bad time of year. Maybe if I went during prime vacation time I would have had more people to StreetPass with.