I've never worn one, but I'm intrigued. I made a return at REI recently and eyed a cute rash guard but wasn't in the mood to strip and try on.
I am in the water quite a bit, but in our own swimming pool, which gets a fair amount of shade. Of course, we are also near the beach and take some beachy vacations, so I can see this being useful. Like Diana, I also think they look pretty cool in a sporty way. And I have a family history of skin cancer, so yeah, protection is always wise.
There is no way you'd see me in one without a bikini top underneath. 32E needs support! Maybe the less endowed can go without.