I'm in my third year with an O'Neill 7000X 7mm neoprene dry suit, having moved up from 1 piece wetsuits +/- overcoats, and I love it for NorCal diving. I alternate between lycra/nylon only and thinly backed polypro (TUSA I think) depending on water temp (high 40s to mid-50s). For diving at the upper range the undergarment is just to keep the cool rubber off your skin. I have a heavier (not heavy by drysuit standards) insulated undergarment, but it's unnecessary underwater so far, and makes pre-dive body temp more difficult to manage - something new with drysuit diving to me, you have to be mindful of temp regulation when out of the water. I've worn a variety of synthetic stuff underneath and any of it is fine, if it fits close enough. This thing has really changed my enthusiasm for cold water diving - I'm toasty from head to toe for the 90 minutes of my longest dives. It's not snug like a wetsuit to start with, but conforms to fit well w/o squeeze and while it's bulkier than a wetsuit, it works well enough for structure spearfishing, where I'm moving quickly, upside down a lot, and reaching into holes. I went neoprene for the $400 price and the thought it would hold up to abrasion and puncture - which it has - better than other suits and would be easier to fix. Other posts suggest that may not have been true. The stiff neoprene reminds me of what was used in suits many years ago, not the ephemeral (but comfy) stuff you find now. I use the Apollo BioSeal - a big soft rubber band - under the neoprene neck seal and it has improved already good sealing to almost perfect. It's $50 but you could make one with a wide strip of thin neoprene and glue. I assume you're a dude - the women's version of this suit, which a friend dives, has wrist seals that don't turn under and she has regular leaks which aren't often a problem for me. Apollo makes wrist BioSeals too which I assume would solve any problem there. The older version of this suit I tried had built-in hard boots, which made for too much dead space in the calf and boot and I really had control problems when head-down - the boot would float off my foot. The current suit has soft feet - you supply overboots - and I rarely have problems with air moving around in the suit when I invert. That's something I've wanted to ask fabric-suit divers - with the extra undergarment thickness, is air movement more pronounced when you change attitude? With soft feet, the suit inverts completely and dries as fast as a nylon II wetsuit.
I can't say whether I'd like a fabric suit better, but other than bulk and extra lead this is much better than wetsuit diving. One issue - you can't pee. When I dive 5 or more times a day in a wetsuit I sometimes get where I need to pee every dive, sometimes more than once. Don't know what causes that. So far I haven't done that many successive dives in the dry suit, I wonder if the lesser skin exposure to the saltwater would change whatever odd physiology is going on.
