DonM A lot of the foam ones are hot.
I did not notice a big difference going from a traditional mattress to the Nectar specifically, and I live in Texas. Was a little warm in the old mattress, still warm on the new one. BUT, we got a light, breathable blanket and lyocell (tencel) sheets and boy, what a difference. House set to 70°, sleeping upstairs where it's probably 4° warmer, I wake up in the night cold pretty often.
Whatever your sleep situation, lyocell sheets and a non-polyester blanket are some ways to sleep cooler.