I have a lowepro minitreker, which served it's purpose well, recently upgraded to a rolling backpack bag, after the addition of new gear outgrew the lowepro, the rolling bag is awesome, beats the heck out of carrying a heavy bag,
as a sidenote, in addition to a good bag, I highly recommend a photographers vest, a vest is the best way to do WDW, everything fits in the vest, is easily accessable,weight is distributed well, so it never becomes a burden, no need to figure out what to do with a bag, when going on rides....
I actually keep all my cards, batteries etc in the vest, the bag and vest go everywhere I go, when I herad out to shoot, I transfer my lenses, and flashes if I'm going to need them, into the vest, put on the vest grab the camera, and lock the bag in my car,
it's so convenient, no need to worry about anyone taking anything out of the bag while I'm focused on shooting a sporting event, or whatever it may be at the time...
If anyone is interested in a good price on the bag, or a vest pm me, I've found a few good sources, with great prices..