Thalassamania:I've never tried a semi-dry in REALLY cold water, how do they hold up? I used to use an O'Neil SuperSuit in CA. But that was an attempt to make a dry suit that wound up being semi<G>.
Hopefully someone with more experience will pipe in. I've had mine deep in a quarry below the 3rd or 4th thermocline several times where the water is around 55 degrees. I could feel the chill creeping in a bit through the suit. It felt good after being hot on the surface, but keep in mind this was 120 feet deep so the neoprene was essentially compressed to nothing. The longest I stayed deep was 10 minutes or so. I didn't get very chilled. Not much to base a decision on here - sorry. Do a search. I believe you'll find people vouch for them in water down to 50 or so unless you dive deep.
I spoke to a cave diver gentleman in Boston a couple months ago. He told me that the isotherm is very popular with cavers because they keep you warm, mostly dry, and greatly reduce drag over dry suits.