I find redosing does work which is a big part of the problem. You never really get back the euphoria of the first coupla doses in a session but you still get high even if you do it round the clock for days (not that I have but plenty clearly do).
It affects dopamine more than serotonin I think (may be mistaken but it certainly has a strong dopaminegic action) which would go some considerable way to explain the strong fiending it produces.
I think this is pretty close to the mark - well, it certainly feels that way. You get the buzz of euphoria to begin with, but then it turns into a stimulant buzz. A bitter-sweet double whammy.
