I've merged these because the original post was just a couple weeks ago, and the feedback is related to your question.
As socal said in an earlier post, bk-MDMA is most similar not only chemically to MDMA, but in it's effects- it just gets less like MDMA from there. If you are truly looking to substitute MDMA, that's as close as you'll get- though many may actually prefer the aspects of the RC "roll" experience over MDMA, they are not less similar. Many enjoy the more ego boosting energetic experience of mephedrone, many enjoy the psychedelic aspects of 2c-i and 2c-b etc. but if you're looking for something close to MDMA, you disappointed with the lighter shorter roll that bk-MDMA will give you.