[Huan]

I have a 6.5mm open cell (maybe 5mm after aging) and use it all year round, I am fairly stocky in build so tend to hold heat better than long skinny mates! also I think you can train yourself to go through the cold barrier, it sounds weird but is more less mind over matter.There are threads alluding to this in the freedive section.
as for the points already made by others , I use 4mm socks I think that they are Titanium lined but not open cell,They seem fine as long as they have no holes in them.
Gloves are 3mm in the winter and 1.5mm neoprene backed in the summer, I always wear gloves as the chances of injuring your hands when soft is high.
I have found that the reusable heat packs are a great job. when you get out.
I have dived down to 8ºC comfortably , not an all day session though!!