i use both MDMA and opioids, and i prefer not to mix them. some experiences have been worthwhile, but in general i think they're better off used seperately.
yes, opiates can be useful for eliminating a bad comedown. i don't recommend doing this too often, however.
no, they're not usually that fun taken together. usually. this is my opinion, of course. some people seem to enjoy the combo of MDMA and heroin, called an 'H Bomb'. yet, many heavy opiate users i know do NOT like MDMA, as they feel they get very little from it.
remember, in the interests of harm reducation it's worth saying that if you use opiates to combat your rough E comedowns too often and this becomes a habit outside of just using them for this purpose, you already know where that could lead. addiction isn't very fun.