Ladies and Gentlemen, LEND ME YOUR EARS. I do NOT want to sell you this product. I have some basic advice that can make it work like a dream. Sellers are motivated by profits so they're never going to tell you the key factor that determines whether this item is a good fit for you. The short version goes like this; If your mattress is saggy, worn, with dead springs then this topper WILL NOT HELP, it will MAKE THINGS WORSE!If you have a firm mattress and are looking for something to add support, it is a DREAM. I began this process with a worn out mattress. So I will share what happened below.At first, I was deeply disappointed to the point of anger. I wanted a FIRM mattress topper to make up for a sagging mattress. I also sleep hot and was counting on the "cooling gel" (or however they market this as reducing heat). Without a cover it's an insulator. As soon as I let it rise and air out, I installed it under a cotton sheet. I laid on the topper and it really felt like a pleasant cloud and it was cool. It was not the firm sensation I sought but I liked it. 15 minutes later and I noticed I was sinking deeper into the mattress than before. Then coolness rapidly turned into a heat trap! It must have similar material as wet suits as my body heat and moisture began to heat the material. Now, I was furious. As I became increasingly frustrated my body got warmer and warmer. All I could think about was "the reviews MUST have been bots!" I also dreaded the hassle of returning it due to the way it's packed etc.I spent hours the next day after a poor night's sleep looking for a Queen mattress under $350 that was extra firm but I didn't trust any of the reviews. I did remember reading one 5-Star review for this product (here) that I believe is up near the top in rankings due to its helpfulness. The person explains this exact point I'm trying to make very clearly. I'm trying drill this point home so you don't make the mistake I made.At one point I remembered that there was firmer queen mattress in my guest room. So I swapped the mattresses and placed the topper. Thats when my frustrations made a full 180°. It may be essential for one to purchase a special cover for this pad that provides a layer for air exchange so that it cools. In my case, a fine sheepskin blanket provided satisfactory airflow to negate the heating effect produced when the body's warmth is trapped in this material.I'm writing the review from the bed. It's ridiculously comfortable. I don't want to get up. Yet, please folks, understand it WILL NOT work to transform a soft mattress into a firm one. Don't make that mistake or you'll be so frustrated you won't have the energy to leave a negative review. You'll make your situation much worse! You'll also miss out on the chance to experience the effect when this product does work which is deserving of all the superlatives in the reviews.I didn't try this but the 3" is dense enough I suspect that placing it atop the box spring would have been better than trying to make it work in with my old worn out mattress.If you can afford to get a very basic firm queen mattress or swap one from your home then this can work. If not, just find a new mattress. There's no method I'm aware of to ressurect a bed where the springs are toast. I'm tall, muscular and heavy as lead. I have a fast metabolism and exercise often. I need a firm and supportive bed that's cool. If one takes a moment to consider that we sleep roughly half our lives, and that so much of our health is contingent on the quality of sleep we get, this is one of the few things money can buy to make a meaningful difference in your quality of life. Not necessarily "this" exact product. I'm sure there are products with far better design and materials where the effect ultimately supports multiple body frames perfectly. Perhaps there is a mattress topper that will work on a flimsy mattress just NOT this one. Yet the potential options I've found are expensive.I'm taking the time to write this to potentially save YOU time and frustration. I'll circle around to the top. I honestly think a piece of plywood would suffice over putting this on a saggy mattress. It DOES add additional support to the contours of the body IF you have a firm underlying foundation.