They don't limit your range of movements as much as a wet suit and you can wear different combinations of clothes underneath to stay warm depending on the temperature.
They keep the wind off as well. If you get a wet suit, you'll need a windproof layer over the top.
Three quick dry suit tips:
1 Always rinse the zip and the seals after use.
2 Always put talcum powder on the seals for storage or useage
3 Wear sailing trousers over the top of the dry suit to prevent it getting snagged on deck fittings and ripping.
If you have Helly Hansen over there, my old HH was the best suit I've ever had. I've had Henri Lloyd (over priced crap), Musto one peice (crap), 2 HH (excellent suits) and my current one is a Typhoon (jury's still out on this one).