As Perrone says, semi-dry=semi-humid. I usually dive such a suit, but under 60-65F I change to the dry one.
About the Mares Isotherm, some friends have been using it, and the most important claim is the plastic dry zipper. It has to be carefully lubricated with a special silicone gel they give you with the suit, but many friends suddenly noticed, on the boat, that the zipper had opened on it's own, on the middle, for example. I don't want to imagine if something like that happened under water. So, the main rule, even for semi-dry suits is IMHO, to buy only metal dry zippers, like the ones made by Dynat.
I repeat, if you're really thinking to dive in water temperatures under 55F, take a full Dry-Suit, it's maybe more expensive but the increase in comfort is unbeateable!
Hidroj