Memory foam is temperature sensitive and reacts to your body temperature, molding to the shape of your body in normal temperature. This minimizes pressure points and distributes weight more evenly. All memory foam however, is not the same and differ greatly in the following areas: response to temperature, hardness or softness, density, and durability.
Generally you should avoid the really inexpensive memory foam toppers or pillows you find because they are usually made of either low density memory foam or are layered with cheaper foam products. You will not get the same comfort from inexpensive memory foam as you will with higher density memory foam.
Typically, memory foam with a 4 lb. density will keep its comfort for a good 3-5 years. Products in the 3-3.5 lb. density range will not last as long as a 4 lb. density or higher. The 5.3 lb. density will provide the most cushion and support for your neck and back pain. The 5.3 pounder is also the most durable.
For individuals seeking a restful night of sleep, with relief of neck and back pain while sleeping, should choose memory foam with a 4 lb. density or higher. This will last the longest, providing you with the comfort you need after a long day at work.