I bought this mattress 5 months ago because I was moving into a new house on a budget. I read the negative reviews about possible chemical smells and hot, sweaty nights, but I took the chance anyway because I never had a problem with feeling hot while I sleep. On a positive note, the chemical plastic smell of the mattress dissipated after I opened the box and let it air out for a few days. Unfortunately, I instantly started waking up sweaty when sleeping on this mattress. When my boyfriend moved in, the heat of two people sleeping on the mattress made it absolutely unbearable.
I've never historically been a hot sleeper, so I purchased a "cooling gel memory foam pad" to place on top of the Zinus mattress. It still didn't work, so I shelled out even more money to buy a plain cotton breathable mattress topper... this is the only thing that gave me any relief. If you insist on buying this mattress, definitely invest in a good, breathable, 100% cotton mattress topper (not the cheap vinyl/terry cloth ones they try to bundle it with on Amazon - they trap the heat).
The only positive thing about this mattress is that it's actually very comfortable to sleep on, especially since I'm a side sleeper. I'm disappointed that memory foam just doesn't agree with my body chemistry.