We bout the 5.5" mattress topper because our *** bed was only 8" high and our sheets no longer fit right. We used the mattress topper on top of our mattress but it negated the benefits of the *** adjustment capabilities. We ended up putting it under the *** mattress and now we have a mattress that is closer in height to our original bed. For the week we had it on top of our mattress it functioned very well although I would recommend letting it air out for a few days before using it. The smell of the memory foam wasn't able to escape under a mattress pad, sheets, blankets, comforter, etc.