I went running on the beach in my Five Fingers yesterday. I ran for 18 minutes and wasn't sore at all immediately after nor the next day. I normally get sore after running for about 12 minutes in them. I don't think that this lack of soreness was due to an improvement in my conditioning but rather in the beach surface. I was running down where the waves break on the shore, so I was ankle deep in water or on damp sand. It definitely had nice shock absorption properties. Also, the air right at the ocean seems a little cooler than it is inland.
Even though I have the KSO (Keep Stuff Out) model, lots of sand got in my shoes. It all fell into the toe holes and I felt a weird pressure on all my toes. I emptied a crapload of sand out of my shoes when I was leaving the beach. I emptied more after I got home and took the shoes off for good, even more after the shoes had dried in the sun, and apparently my mom got a bunch of sand out of them this morning. I didn't have any sort of skin irritation/blistering even with all that sand in my shoes. I would like to run barefoot on the beach, but I'm scared of cutting my feet on sharp rocks/shells (or soda cans).
I would definitely do this on a regular basis if I lived near the coast.