Ah yes, SpeedyG, Mirtazapine is an a2 antagonist, not an agonist, but this doesn't explain its effective anxiolytic effects at sub-prescribed dosages, but it is also a 5-HT2C antagonist, 5-HT3 antagonist (anti-emetic), and contains other actions that are unique to antidepressants.
Mirtazapine is the only Rx antidepressant that I would actually consider taking at full dosage one day in combination with low-dose daily clonazepam if my anxiety ever got that bad, for now it's just low dose mirtazapine (3mg/day). SSRI's, on the other hand, I believe work best in in conjunction with microdose amounts, so I'm combining the 3mg mirtazapine with lexapro (1mg), and zoloft (3mg/day).
My main AD is DXM, which I take daily at 60-90mg/day amounts for a year now to prevent opiate, benzo, and amp tolerance from rising, and also of course for its AD and anxiolytic properties via NMDA antagonism and sigma agonism, so take that in to account since it also acts on the SERT as do SSRI's.