I have not used a zip-T, but I know some people like them for ventilation management.
I have been using a Smartwool l/s microweight T for almost a year and bought a s/s one for the warmer summer months, mostly due to the positive comments here at BPL. I am glad I gave them a try. They can handle a wide range of temperatures, and I have found I can use a combination of the l/s shirt and a windshirt comfortably to temperatures in the low 30's while I'm moving.
Yesterday a few of us snowshoed all day in temperatures hovering in the mid to upper 20's, and I was fine in the T, a fleece vest, and a windjacket. They wear well, although my shirt has a few small holes in the midriff section and sleeves from when I did a face plant and 30' slide down steep, wet heather this summer. That was due to user klutziness and no fault of the shirt. I am definitely a true believer of the lightweight merino shirts, year-round.