I have zone envy myself. That photo is from my house in Virginia, which I moved out of a couple months ago. I certainly won't get a rosemary like that in Michigan!
We were in zone 8 there, so we would get freezes, but never as low as 0F. You're right that drainage is key - I was able to keep it in the ground mostly because my soil was pure sand and didn't hold water at all.
Joanna, I'd be anxious to hear if your rosemary makes it. When I got mine, all the info I could find said they were only hardy to zone 9, yet lots of people in my city had them (and even bigger than mine!). I wonder what their real hardiness is
I would get a bit of die-back if we had a hard freeze after it put on new spring growth, but otherwise it never seemed bothered by the winter weather.
