I'm just starting on mirtazapine, I'm coming off amitriptyline at the same time.
I was on 150mg of amitriptyline but now I'm on 100mg and 15mg of mirtazapine.
I have to stay on this for a week and then go on to 50mg amitriptyline and 30mg of mirtazapine for another week and then go back to the doctors.
I've not had much of a problem apart from being sleepy and last night I had a bad ibs attack with 5 hours on the loo, but that happens quite often anyway so I can't nail that down to mirtazapine.
Hope this helps me as I've been steadily getting worse with depression and anxiety while on the amittryptylene.
A lot of my anxiety is caused by my aspergers so it'll never truly be fixed by tablets, but if it dulls it I'll be happy.

Caz xxx