I was on Mirtazapine for several months.
The weight gain is due to you feeling hungry and craving carbs, hence you eat more. Th drug doesn't change your metabolism or anything and the weight gain can be avoided by being disciplined and strict with yourself, ignoring the hungry feeling - it can be done, but it;snot easy, especially if you're feeling low.
As for sedating side effects - this drug is marvellous for helping you sleep and treating anxiety, absolutely marvellous. The first few days will leave you feeling quite sedated, however you do get used to it and eventually don;t really notice it, although you are aware that you're feeling much calmer than normal and a little 'heavy' in the head - perfectly tolerable, but you;re always aware there's something chilling you out.
Interestingly, lower doses have a greater sedating effect than higher doses. I have tried this and can conform it to be true.
One tip.... when you wake up in the morning, force yourself out of bed and into the shower, or some other stimulating activity - exercise etc. The worst thing you can do is stay in bed, as you'll never fully wake up and can drift off again very easily and stay in a semi-conscious state for hours, with unbelievably weird dreams.No matter how much sleep you have, you'll never quite feel fully refreshed.
For me, in my experience, I was suffering crisis levels of anxiety where my gums were bleeding, my teeth became lose and I hadn't slept for almost a week - the Mirtazapine sorted me out instantly, so for that, it was a wonder drug. It didn't however touch my depression.