MDMA, used correctly, is very safe. There is some debate about this, however...
I heard it causes brain damage!
The term normally used is "neurotoxicity". This sounds a little scarier than it actually is; the term simply refers to physical changes occurring within your brain. There are many things that cause physical changes to your brain; most of them do not qualify as "brain damage."
Then what does neurotoxicity really mean for me if I choose to use MDMA?
MDMA is much less neurotoxic than some currently available prescription drugs. The debate as to whether the effects it can have on your serotonin receptors and whether they are permanent is continually underway; however, hundreds of studies have been conducted over the last twenty years, and none of them give any conclusive proof that long-term MDMA usage is harmful to humans. For detailed information about studies related to MDMA and neurotoxicity, go to the MAPS website.
I heard it causes your body to overheat and your organs to shut down!
MDMA does NOT directly increase body temperature. MDMA does NOT cause your organs to shut down. What MDMA does do is interfere with your body's ability to regulate its own temperature. What this means is that rather than being the constant 98.6 degrees F that it is supposed to be, your body temperature will be dramatically affected by your surrounding temperature. If it is very cold, your body temperature will drop lower than usual. If it very hot, your body temperature will be higher than usual. If you are involved in a very physical activity, such as dancing, your body will also generate more heat, causing the same effect as being in a hot environment. Your body needs water to help cool itself, so be sure to stay hydrated. If you are in a hot, indoor environment, try to take a break outside as often as possible. If you can't retreat to a cool rest area, consider limiting your dancing or leaving the party. Most if not all cases of MDMA related heat stroke have been a result of overcrowded venues with insufficient ventilation and/or not drinking enough water.
Okay, what I really want to know is - is it safe?
Given the lack of conclusive evidence regarding neurotoxicity and the fact that people have been using it continuously for ten, fifteen, and twenty years without any noticeable negative effects, it's fair to say that the answer to this is a firm YES. It is, however, like anything in life: do it cautiously and with thought. Make sure you get true MDMA and not an adulterant. Make sure you regulate your dosage properly, as well as drinking enough (but not too much) water and regulating your body temperature during your roll.