Mirtazapine is an interesting antidepressant - as rhun said, it's a good sleep aid that provides a mood boost the following day. Unlike other antidepressants, Mirtazapine doesn't primarily work as a reuptake inhibitor but as an antagonist on various "bad" receptors - blocking these improves your mood by allowing your brain to release more noradrenaline and dopamine(!).
But yes, it is notorious for causing weight gain - still, after doing a successful in-patient therapy for my depression I was easily able to lose the weight (despite still being on Mirtazapine) since not being super-depressed gave me the energy boost I needed to hit the treadmill, and resist the urge to binge on cereals.