When used properly and safely, MDMA isn't that bad for you. It's still not great for your body, but as long as you stay hydrated and don't mix it with thing it's not supposed to be mixed with, using it once won't cause any problems. I suggest that you read the available literature (erowid.org is, as always, a good place to start) and decide for yourself if it's something you're willing to try. For those that decide that MDMA is safe enough for their personal standards, I recommend trying it at least once; rolling is a much more interesting experience than most people expect (it's more than just being really happy for a couple hours).