I haven't used the frogskin under my wetsuit yet. I have a good 5 mil that keeps me warm enough for most diving, and a 6.5 mil semi-dry if I am going hard-core. So, I don't really need to layer. I can tell you, though, judging from how well the frogskins heat up the water next to your skin, that they will be a tremendous underlayer. I think this is due to the fleece lining. As I said, I am thinking about replacing my 5 mil with the Probe 5 mil semi-dry with the same "quick dry" lining.