Hi - just thought I would share some hopefully reassuring thoughts.
I was mirtazapine for several years for OCD and anxiety until recently, after my anxiety deteriorated after a sudden, unexpected bereavement. It was my doctor's idea to change, and I did what I was told. I lasted a week on the new meds before the severe anxiety and other side effects became too much. I am now back on mirtazapine (one week and counting), and already feel so much better. My anxiety levels have returned to pre-med change, which while are higher than I would like, it is much better than a week ago.
In terms of start up effects, I felt dizzy for a few days and a little sick off and on. But now I feel ok, and am sleeping through the night and not waking up with panic attacks. The daily panic attacks have been replaced with milder anxiety attacks, which means I can wait for my new counselling appointment to come through without feeling like I am going completely mad.
In other words, those who are worried about starting - it will be ok. Watch what you eat and remember that you will be dizzy at first. But it will ease quickly.