Memory foam mattresses are very comfortable but they can get incredibly hot in summer and tend to retain your body heat. I'm not sure whether a memory foam mattress topper would have the same effect. It's not so thick as a proper mattress but I guess the heat retaining properties would be the same.
I once spent several nights on one of these mattresses in a heatwave. It was like sleeping on top on an oven and whenever I rolled over I took all my body heat with me. For that reason I would never buy one. Certainly if you're somebody who sweats a lot or generates a lot of body heat I wouldn't recommend one. And if you're a menapausal lady suffering from hot flushes they definitely a big NO NO !!
We tried one and in about three days we had to take it off as we were burning up !!!! They just get incredibly hot and you feel as if you are on an electric blanket which is set to max. I also feel the cold a lot so its not as if I am a warm blooded person.