I have a 4/5mm Ralf Tech semi-dry and a 7mm Waterproof semi-dry and they work far better than a similar-thickness simple wetsuit.
As someone said, semi-dry is semi-wet, but I find you don't get the flushing you do with a traditional wetsuit thanks to the double seals on the wrists and ankles, and the longer neck seal. When I was filming in South Africa, the water was 16°C and we were static in the water a lot of the time, and when we came out and took the suits off (we were in the Ralf Techs), I was virtually dry underneath, just slightly damp, and that was more from sweat than anything!
The good thing about them is that if you get hot, you can always pull the neck to allow a bit of cool water in to cool off, whereas if you get cold in a normal 5mm, there isn't a lot you can do to warm up (unless you drank lots of water before you went in...)
Mark