They are noticeably sweatier than the standard covers, but not a deal breaker for me. I don't really notice it until about the 50 mile point, and then only on the really hot and humid days where I am stubbornly leaving the a/c off. Balancing that, they improve the overall comfort of the seat considerably. There is a fair amount of extra padding here and there, and for me it makes all the difference.
They are also much easier to clean than stock. In a previous post I mentioned the WO shampoo, but failed to mention that it is used very dilute. I put a tablespoon in a gallon bucket of water and slop it on with a rag, then dry with a towel and air dry overnight. My experience is with substantial dog drool (in my wife's Prius backseat, no dog in the front seat of the Insight), but a co-worker reports easy cleaning of hot fudge sundae and other food products from her Prius WO covers (four kids under 15).