Got my Rainbow's today. Ordered direct from rainbowsandals.com. The men's small size (7.5-8.5) seems to run a little small. I wear a size 8, with a narrow foot. The small's fit, but barely. I'd prefer if they were a half-size larger, but they'll work. If you wear a 8.5, I would definitely go with mediums.
Unfortunately I was not happy with my expensive sandals.
Anyway, I only wore them a few times. They shrank dramatically, bowed and turned into unwearable flip flops...see the pics. They won't go back into shape and are incredibly uncomfortable now. One of the reasons I was prepared to buy these expensive sandals was because they had a lifetime warranty/guarantee. That failed too as their customer service rep told me they wouldn't honor the warranty and indicated to me I shouldn't have had them in the sun and/or gotten them wet in the first place.
Fine, maybe at least mention that somewhere on Amazon or on the product page.
Moral of this story is where everyone is talking about the lifetime warranty, make sure actually go to their website and read it in detail.