I will absolutely agree with Brat17.. Weight gain, constant eating (even things you don't like) and a big increase in libido. Some of those things good/some bad!
I was on Mirtazapine for about 9 months, I was 7st when I started (I'm 5"6') and 11st when I finished them. They stopped working for me in the end which was disapointing, as they had helped me sleep and calmed me down. They also gave me mood swings, which I have now discovered is my reaction to a lot of anti-depressants. I would go from dancing/twirling in circles and giggling (very strange for me) to sobbing and wanting to die.
I started on 30mg on a night, then added 15mg on a morning and the 30mg at night, then that got changed to 45mg every night. It stopped working after about 5 months and apparently 45mg is the max dose. I wish it had worked for longer for me, although I would be the size of a small house by now!
Hope this works for you