I've got a custom Harvey's Dri-Flex Two-Piece Wet Suit. On many dives, I'm the warmest one of all. (Might have to do with my body, too...as I'm a furnace.) Harvey's makes a great suit. Yes, it's quite flexible, so it's easier to get into...but I haven't noticed any difference at depth either...
I've been to 47 degrees with it and been fine.
As many people have already mentioned, the biggest issue is fit. It has to be tight to your body or else water will flow in/out, causing major heat loss. I'll take my suit off after a two dive day and still have dry spots on the inside of the suit.
The biggest issue that may force me to go dry is value. Meaning this: Wetsuits will only last for so many dives. Each dive, they get compressed, and over time, they lose their ability to insulate. I dive a lot, so I don't want to be buying a new wetsuit every year or two. A good drysuit will last much longer. So even though it might be more expensive up front, it's a better cost/dive value. Something to consider if you do a lot of diving.
My two cents.