HI Heather,
I've used them since mid april every day on my morgan who strained an extensor tendon when he slipped on grass trying to run from a helicopter (in a 12x24ft pen mind you).  The first day I did as instructed and didn't leave on for more than 4 hours and his leg was definitely hot under the wrap at the end, but after that I've left them on for up to 40 hours at a time and the leg isn't that much hotter than under a regular standing wrap.  This is during CA summer (110F) and dry heat but he didn't develop any fungal or other skin conditions on that leg under the wrap - and he usually does get scratches on that leg as it's his only white one.  They are expensive but after 6 months of use, rotating between two wraps on that one leg so I can clean them regularly, there's little sign of wear.
Josie
Davis CA 06/09