I really wanted to love Birkenstocks but I got terrible blisters under my foot and it took me ages to break them in. Then I was able to wear to throughout the summer...
Next year i had to start over and it took ages to break them in, so I won't be taking them to WDW this year.
I also tried Fit Flops and didn't get along with them, think I may have difficult feet ;-)
I got some Crocs(gasp) a month ago, the shayna style and have been wearing them lately and they're pretty comfortable. Will try and properly break them in for the next 4 weeks
I'm usually a size five or a six depending on the shoe, so the first pair I ordered which was a size 5 (38) slipped off my feet as they were a little small so I swapped for a size 6 (39) and they were perfect.
The rubber straps across the top moulded to the shape of my foot perfectly so that they never slipped off at all. We did a lot of walking over the two weeks!
Try them around the house for a short while to see if you like them, and send them back if you don't