In contrast to many prior studies, ecstasy users in the new study showed no signs of cognitive impairment attributable to drug use: ecstasy use did not decrease mental ability.

The resulting experiment whittled 1500 potential participants down to 52 carefully chosen ecstasy users, whose cognitive function was compared against 59 closely-matched non-users, with tests administered at several stages to make sure participants were telling the truth about their drug and alcohol use.

Science Daily: New Study Finds No Cognitive Impairment Among Ecstasy Users

A small sample size, but this study does not make the same errors that many other studies did.