I just returned from WDW and used the Lowepro Slingshot AW300 while I was there. It is a big bag, but worked out much better than I expected. I didn't have any problem fitting it on the rides - in most cases it just went on the floor by my feet. Two notable exceptions are Soaring and Mission Space. On Soaring, not much of a bag is going to fit in the little basket under the seat. In this case I just left it on the floor in front of me and the cast members moved it to the side where I picked it up when the ride was over. On Mission Space I was able to fit it in the baggage container, but it was a major cram-a-thon to get it in there. I'm not sure if they allow the bag to ride on the floor on this ride as an alternative.
For background, the main motivation for this sort of bag is to fit a DSLR with a 70-200mm lens attached - it even fits this with a teleconverter fitted. As an alternative I tried a smaller bag and carried the 70-200mm separately and quickly learned the error of this when it began to rain. It's much easier to shove the camera + lens into the AW300 instantly rather than running for cover and then trying to disassemble the camera so it can be put into separate the bags. Being able to stow the gear without disassembly is also important if your family follows the Pirates Code - fall behind get left behind.
The AW300 has a weather proof cover that can be pulled out to cover the bag. I got to try this out several times - in one case we got hit with a major downpour during Illuminations that was so bad that things got knocked over by the wind. The camera stayed dry though.
The AW300 is a big bag and I don't recommend that it be loaded down with everything you own if you are going to be in the parks for the whole day. Rather, it is better to just take what you need. Nevertheless, my shoulder did get a bit sore by the end of the day, but the waist strap will take a lot of weight off the shoulder if used.
On days when I didn't take my big lens, I used that spot in the bag for a water bottle. There is also plenty of room for a hat, sunglasses, and a rain poncho. On some days, while the bag was big, it was quite light.
In case anyone is wondering about the motivation for carrying a 70-200mm, it was mostly for Animal Kingdom, although it did come in handy at some of the live performances too. Fun to use, but not something to carry all the time. A good superzoom lens and a light fast prime should be sufficient to cover most situations.