I would never wear them out in public, but I like to buy the $1 ones you can get at old navy, and wear them around the house, or to take out the trash.
OH and yes, I think it's appropriate at the beach, but that's pretty much it.
I hate the flipflops posted above tho, I only use the ones with the thingie between you're big toe and the one next to it (idk what they are called, embarassing)
Im not exactly looking at peoples feet so it doesn't bother me. I wear them all the time in the summer cause they're convenient. Only do I notice when people wear them with pants, cause it looks so weird.