A couple of years back, I was on Citalopram for 12 months, which was then changed to Mirtazapine for a further 6 months.
It was a smooth change over - so I can't really help you with how long Mirtazapine takes to work. My guess would be within 2 weeks.
The most memorable side effect of Mirtazapine was vivid lucid dreaming - which was quite surprising, but actually quite pleasant. Also, my appetite and weight increased - which was probably not a bad thing in my case.
You will find a list of possible side effects here: