I think the thong sandals are great and I have worn them quite a few times to WDW. I also like the brand NAOT, which is similar to BirKs but a little more narrow
I love my Birkenstocks and I do wear them all day in the parks. If you have never worn them you want to purchase them and wear them lots before the trip. They take a while to get used to and your feet may hurt a bit for the first few times you wear them.
I love my Birks but I only bring them to Disney to wear to dinner or around the resort, not all day in the parks. They can't get wet and I prefer something that can get through anything the day brings. Birks are a nice second shoe for the later part of a Disney day for me.
I wear mine in the parks and never have problems with blisters or achy feet, legs, etc. I choose them over my running shoes! I wear them almost every day at home though so they are very broken in. I don't have the ones that go in between the toes - I think that could hurt after a while. I have the ones with just one band. I think it is called the Madrid.