Well they are black, so they can get hot. I keep the top up with just the windows out in the summer, bikini style. I normally sweat a lot but don't find the covers any worse than any other seat in that regard.
About the Neoprene - Possibly there are different kinds of Neoprene but my wetsuits are not water proof. How long does a wet suit stay wet after you use it? Mine stay wet for at least a couple days. Since I haven't had Neoprene seat covers I guess I shouldn't comment on them, but anyone who says wetsuit Neoprene is water proof is just wrong or has a different definition of waterproof.
Well, not to get off-topic, but most people sweat a lot with any wetsuit on so it is going to be "wet" against your skin no matter what... It doesn't allow the outside water to transfer through to your skin and subsequent heat-loss thereby keeping you warm.