Significantly more comfortable. None of that damp that cotton t-shirt & shorts leaves you with. It's much easier to get out of leathers after a hot ride as well.
I'd forgotten how good they are until going out on the hants rideout on saturday with t-shirt and shorts under my leathers as the undersuit was in the washing machine.
IT was entertaining trying to get out of my leathers when I got home that night. At least my wife found it funny...