Drysuits are I think able to keep you warm, or cold. It really depends on your undergarments.
For me my undergarments are an Andy's US-35 and a layer of fleese over that, probably close to a 200 weight fleese undergarment. Many freeze to death with that combo in the temps we had this weekend. I'm told that the type of drysuit also makes an impact, things like the neoprene drysuits tend to keep your warmer and a trilam is colder.
That said, I started diving drysuits because I ran across a good deal at the LDS and I figured it would help me with the surface interval more than anything else. It does help that as I thought it would so I'm happy.
I find the idea of being totally dry nice. I like not having the neoprene smell on me and not having to dry off and then wait a day or so for the wetsuit to dry. When I exit the water by the time I get into the vehicle all my gear is dry and ready to be put up, nice feature of some drysuits.
Of course there are more issues with drysuits, and the skills are a bit different. It's not that much harder or anything, just different and more expensive.