I have had a Gardman mini greenhouse for several years (3-4?) and it has held up just fine through the harshest of winters. I have not tried overwintering plants in it because it would be a pain to keep the plants in it watered; i.e., mostly I overwinter seed trays and they need natural conditions, so it's easier to leave them outside.
But, as far as durability of the cover goes, there's no problem.