Memory foam here although the guys who picked up the first one in less than a year called it "garbage foam" because that is where they haul the returns I guess :-( This one is still OK at nearly 3 years old.
We love sleeping on it and our backs are happier than on the old regular mattress. It's not really good for sitting on but you are not really supposed to sit on any of them.
I get hot so much that I just say my thermostat is broken but I have been fine with memory foam and we use latex foam pillows which I love. I don't think it is really makes me any hotter.
There is a big difference in how it feels at different temps though. But when it is really hot or really cold out the HVAC is doing it's job inside :-)