A bit too firm.
Maybe it takes a while to break in, or it's because I have a dust mite cover and a temperature control mattress pad on the mattress, preventing my body heat from softening the mattress, but the mattress is a bit too firm for me. Definitely not 'plush'. I don't have back problems, and after a few months sleeping on the mattress I still don't have back problems, so I can't speak to how 'harmful' or 'helpful' this mattress is on your back. Hopefully it'll soften up in a few months, and if not, I'll get a nice down mattress topper for it. It works great on a platform bed, so it definitely does the job.