I dive wet - I guess that is what people are calling it. I have used the 7mm farmer John as well as the standard 7 mm with a 3mm hooded vest. The farmer John set up gives you more layers around the torso at 14mm whereas my 1 piece nets me 10mm. Boots and gloves are 6mm.
I've not logged as many dives as the other folk here, but 90% of my 50 or so dives have been in cold water whereas the other 10% have been in Hawaii. When I was diving off the Cypress Sea last year the water temp was around 49 degrees at 94 feet. This past Saturday I was at Lover's Point and the water temp was 54 degrees or so at surface and 53 or so around 45 feet.
I can dive like that for about 3 dives then I get exhausted. I only really noticed the cold during the second dive Saturday but I think it was because I was in the wet suit a bit longer after the surface interval, etc. By the time we got back in, swam out to one of the bouy's, hitting the bottom I seemed to notice it a bit more. But it kicked in again and I was all good.
The worst part about diving the wetsuit for me is that initial cold water that forms the layer between the body and the suit. After that though - I'm typically good. I would say if I was doing this a couple times more a month I would invest in a dry suit. As it is now its maybe every other month and I'm still learning. No need for me to complicate it.
I agree with Chuck in that if you are cold, or prone to getting colder, get a thicker hood, gloves, and boots. Especially the head piece. Heat escapes the head, no? So might as well invest in something a bit thicker.
I won't push any brand but the farmer John was a off brand from my local scuba shop - i think it was actually called Monterey - go figure. My current setup is the 7mm from aqualung.