There was a study published recently that concluded Mirtazapine has no serotonergic action, which affirmed my feeling while being on it (as swim's feeling as well apparently). It doesn't feel serotonergic at all, it doesn't induce that blanket of happiness and the anxiety reduction like the SRI's do. Made both my depression and anxiety a little worse actually.
Makes you sleep easier and longer and will likely make you eat more, both of wich contribute to feelings of happiness. (this is why it does work for people imo) I wouldn't consider it pro-sex neither but people have differing reactions there.
Always worth a try but it get's my vote for worst antidepressant on the market