My mattress is fairly new but I bought one that was medium firm to avoid sinking into it. But somehow the flatness of it hurts my back. It also ends up still being too firm I'm places. After 3 months I had to get something. Several of my friends recommended a memory foam topper. 
This one meets and exceeds my needs. It's cool. I was worried the fabric wouldn't ventilate well but the temperature is perfect. It provides an adequate amount of visioning without being too firm. It hugs my body enough that it makes a huge difference in the way I sleep. I can sink in just enough with gentle cushioning. My back pain Is gone and I sleep more thoroughly during the night. It's not so tall that it interferes with a standard deep pocket set of sheets. 
I've seen similar models and they run much more expensive than this. I don't feel like this compromises on quality at all though. It looks and feels like sleeping on a cloud.