Me and my friends absolutely love MDMA, however we all have concerns with it and have done the appropriate research - Heck, why would I be on /r/drugs if I didn't want to learn/discuss things, and Molly is my #1.
It's amazing, however if you abuse it by having it too often it will become less so. You'll need to take more, and feel it less. We all do it together about once a month on average, however during holidays and in the past we've gone a little crazy. One of my friends have taken a 0.6g bomb of MD, reportedly he's never felt better but the literal steam coming off him was a concern, as well as the inability to keep still AT ALL - Arms, legs, eyelids, jaw, shoulders, hips; all going absolutely crazy, even though I love the drug, I wouldn't go that far and him and some of my other wilder friends seem to have no concern about how much they are actually taking, multiple grams. No-no.
Small doses can still give you the full positive effects if it's proper stuff, and the more you take after the first the less it will hit you; you don't have to take more at 3am just because others are still having fun.
How do I use molly responsibly and not go overboard? Simple. Only take it when you know other close friends will be as well - this pretty much just stops you from thinking it's the kind of drug you can take willy nilly; however if you haven't taken it in a while and are planning to with friends then you can do a lot of different things and still have the time of your life. I'd say once or twice a month is the maximum I'm comfortable with, it's not like I haven't stuck to that but if you find yourself taking it for a long period of time continuously or a lot in a short period of time then you should consider cutting down or substituting it with something else.
How will I know that what I am receiving is the real deal? Me and my friends buy MDMA in crystal/powdered form, from a reliable dealer. If it comes in pills there's pretty much always going to be something dodgy in there you really don't want to be taking; MD has bad side effects as it is but in my eyes they are worth it even just for the life experience and memories. You can buy tester kits online, if you test the powder and it's not MDMA then either stop taking it or find the real stuff.
How often should I use it so no adverse effects come up? Personally I've only had a significantly bad come-down twice, all the other times I felt like I could shrug it off because of the amazing night previously. Any use at all runs the risk of comedown or at least short-term side effects. Long-term effects should only result from heavy/&/constant use, once a month is a good boundary (make sure that's what it is, and not an excuse. 'I haven't taken molly this month, guess I get to' - No.). Limit your dosage as well, I can handle a full gram in a night however I would be happy with 0.25g.
Where should I do molly? On a prepared walk with friends and a portable speaker. Have had the best heart-to-hearts in my life on these (not alongside dangerous rivers, dark areas with no flashlight, make sure you wont run out of water, everything really) At home with only a few close friends, good speakers, simply do whatever pops into your head (while making sure you won't be allowed to do something dangerous/stupid). When this has happened I've painted, ran around the garden, group ChatRoulette/Omegle/Skype - Whatever you feel like doing will feel great. *Try body-gurning. In bed, fluffy materials, make sure you're the perfect temperature and just stretching/exploring the positions your body can make feels absolutely sensational.
If you want to ask me anything I would happily answer.