This is an unusual antidepressant with distinct advantages and disadvantages.
By virtue of effects on complex regulating receptors on serotonin and norepinephrine receptors on their respective neurons it has effects similar to SNRI antidepressants (Effexor, Cymbalta and Fetzima). In addition Remeron (mirtazapine) due to effects on other receptors can be very sedative so it can be a very good sleep agent. Therefore for a patient with bad insomnia along with depression Remeron can be a good choice. However all of this is also double edged. Frequently Remeron can be too sedating.
In addition it has a propensity to cause weight gain, a very unattractive feature which makes it an unpopular choice for many unless they are really struggling. On the positive side it like Wellbutrin does not cause sexual dysfunction. Finally Remeron due to its own separate distinct mechanism of action can be combined with Wellbutrin, Effexor and possibly SSRIs to produce more intensive antidepressant effects if depression is particularly severe.