I've had clonidine scripts, and currently I am on tizanidine, and I too, take opiates regularly, scripted DHC, not as potent as oxy, sure, but nevertheless if I fail to take my dose regularly, withdrawals, and not pleasant at all.
Clonidine is potent stuff, both it, and tizanidine are alpha2 adrenoreceptor agonists, which means they basically 'trick' the brain into thinking there is too much adrenaline/noradrenaline being released, and then it acts accordingly to reduce release of those neurotransmitters.
This both lowers blood pressure, and makes one sleepy, I have a benzo script too, for nitrazepam, a pretty strong hypnotic benzo, and damned obstinate insomnia, but I last used the nitrazepam more than two weeks ago, most of my scripts just get saved up and stashed, so I always have them available if I need/want them, and not used. Every night, a plugged dose of tizanidine knocks me out in just about the time it takes to have a last cigar/rollup, after that, out cold.
With larger doses of either, orthostatic hypotension as has been said, becomes an issue, get up too fast, and back down to earth you will come.
I find both drugs do relieve opiate withdrawal, although it takes a MUCH higher dosage orally than it does rectally or insufflated. I take it usually during the day for autistic sensory overload stuff, but to make me sleep, it works great also (tizanidine, although clonidine does likewise) relieves the hyperstimulation of WD, as well as restless legs, insomnia and sweating.