Have been using a drysuit this winter, water 11 degrees air 9-12 degrees, no idea of windchill factor, I am a wimp in the cold, i usually wear the drysiut with shorts and T-shirt underneath, am always hot.
some hints on suit :
Mine is a loose fitting lightweight Goretex (new type on test) breathable.
Zips : back-shoulder zip is a pain to get into by yourself, mine is has a front diagonal zipper (easy entry) which is fine for seat harness, but not for waist harnesses.
Seals : Work fine, but they need maintainance with silicon. Ankle and wrist seals MUST have a synching overseal (with drainage), otherwise water will get in. I tend to use high ankle boots to form a labyrinth seal on the ankles. The neck seal will streatch slightly after a few uses, don't trim the seal until it is fully settled-in otherwise it will end up too big.
Goretex : get breathable, I still sweat inside mine, if its non breathable it will be a pig.
Cordura : you'll need knee patches and bum patches on the suit, the knees on mine would be worn out if not for the knee patches.
Pee zip : never needed one, would intefere with my seat harness.
Gen info :
The drysuit needs some experience to get into quickly, but once you have the technique it is as quick as a wetsuit.
The loose fitting drysuits give you unlimited flexibility, much more than a wetsuit.
In the water, the drysuits don't hold water, so you weigh considerably less than in a 4/3 wetsuit...good for going higher.
Mine has never leaked, even after big spills.
Getting the air out of a suit with a harness on is a tad difficult.
Swimming with the suit is no problem, its baggy but not an obstacle to movement.
You can keep the suit on all day and you won't get cold, I used to suffer from cold when assisting on the beach in a wet wetsuit.
Taking off the suit and being warm/dry has no comparisson.
Everyone will take the piss, they're jealous of you being warm, but will take the piss on you being a wimp and "lame". You can't look hip in a drysuit.
Don't get a bright colour, piss extraction again. Good for safety though, I have been spotted and identified 1-2kms out at sea.
DON'T FART...you'll look like a Michelin man and it can be unpleasant when you finally take it off.
After a lot of research (I could only view before I bought so had to go with a gut feeling), Kokatak and Stohlquist are the best quality I found but are both expensive, Pyro looked good but not breathable, oneill was neoprene so I couldn't wear undergarments on legs, local aussie maker was cheap but not breathable.
I love mine, I wouldn't go back to a wetsuit for winter.