I'm currently on Mirtazapine but only 2 weeks so far.
I've switched from Sertraline to Mirtazapine because the Sert just stopped me from sleeping and did absolutely nothing to improve my mood. In fact, since I wasn't sleeping it probably made my mood even worse. My first dose of Mirtazapine helped with my sleep. You need to take it before bed because it will make you drowsy- the smaller the dose, the more sedating it is (weird, I know). For me personally, it hasn't made me so drowsy that I can't function. I just take it before I go to bed, wake up when I need to, have some coffee and breakfast and feel fine. However I do know some people that haven't been able to keep their eyes open the next day, so be careful and don't do any driving until you know how it affects you.
Overall I feel it has improved my mood but that might just be because of the improved sleep, since I've only been on it for 2 weeks.