This is an update of a previous review. I have experience with memory foam mattresses and generally knew what to expect, but it seems from the most recent negative reviews that Zinus is having a quality control issue. I'm not sensitive to smells, but the odor has honestly been overwhelming, enough that I've been leaving windows open in 15 degree weather (there goes my cost savings). Many negative reviews agree, others don't mention it at all, so something is inconsistent. Maybe it's just the 12"? After a week, mattress has maxed out at 10-11". I don't care so much about the height, except that I paid for it. Slept on it last night for the first time. Odor hasn't dissipated completely, but wasn't terrible with the mattress pad & sheets blocking some of it. It was firm. Like, really firm. My husband and I have been sleeping on our very firm innerspring mattress on the floor while we were waiting for the odor to go away, and this was firmer & more uncomfortable than that. Woke up multiple times having to adjust due to joint pain. And to my surprise, it did sleep hot. I didn't have one of those plastic-y waterproof covers on it (that I think is sometimes mistaken for the mattress holding heat), but I woke up sweaty a couple of times nonetheless. As an FYI, I'd like to point out a few things. 2 layers of this mattress are the egg-crate type foam, which I believe is notorious for its poor wear. I can feel the big gaps on the edge of the mattress from it and am almost certain this is causing the early breakdown/sagging many reviewers are mentioning. I would not buy a mattress with that type of foam again. I wasn't paying close enough attention to the photos when I made my purchase. Also, I noticed the reviews here are for *all* sizes/thicknesses of this mattress, which is misleading. A heavy person's experience with an 8" twin is not going to be comparable to a petite woman's experience with a 12" king, so read reviews carefully. Some of the reviews mention mold. I read while researching that these mattresses can be prone to moisture build-up & mold if used on a solid-surface platform with no ventilation. If it arrives moldy, that's one thing, but this may explain the people that have it for a year and then notice an issue. While doing research, I came across the suggestion to buy a good quality (not necessarily expensive) firm innerspring mattress without a bunch of extra cushioning and use a good quality memory foam topper with it. This way you get the longer-lasting support of the springs, but the comfort of foam that is replaceable, seeing as how all memory foam weakens over time. I really think this is probably the way to go. At the moment, I thoroughly regret buying this mattress.
Update 8/10/18: Have had the mattress about 9 months now. The smell eventually did go away completely, but it took 2-3 months. Just hope it wasn't toxic. Now in the last month, we've started to experience the breakdown in the foam mentioned earlier in my review. It's gone from rock hard to forming a squishy depression where we sleep, which is a bummer because it actually had become quite comfortable with the addition of a good quality topper. It will have to be replaced sooner than later. Actually that's OK, because shortly after writing my original review, it was brought to my attention that the inner flame-retardant layer of the mattress is made with fiberglass that is known to degrade and work it's way through the cover. If you buy this mattress, NEVER remove the outer cover or you will scatter glass fibers all over. Also, make sure to use a waterproof cover, because I've read moisture makes the fiberglass degrade more quickly. Better yet, just don't buy this mattress :(