ILD (Indention Load Deflection): The level of firmness or softness of a piece of foam (or load-bearing characteristics). ILD is defined by how much weight it takes to compress a sample of foam by 25% (Ex: If it takes 15lbs of load/force to compress the foam sample by 25%, the foam has a 15ILD rating). The higher the ILD rating of foam is, the harder the foam.
On average, the higher the memory foam density is, the better the durability / long life of the mattress. Mattresses with high-density foam can generally be expected on average to remain supportive and comfortable for around nine years. This is only an average, however; the long life of a particular mattress can vary from the average considerably.
Memory foam mattresses with low and medium density tend to have a shorter lifespan, often in the 4-8 year range. Again, this is only an average, and the life of a particular mattress can vary from the average Considerably.
In my opinion memory foam is for comfort. You need a strong base whether coil or foam and then get the right comfort for you. On an 8” mattress, you should look for at least 3 inches of medium to high-density memory foam. 3 inches of memory foam gives you a proper amount of material to cushion and conform to your body. As this contours to your body, try to make sure you get the best active support you can underneath. To maximize its conforming ability make sure the protector does not constrict its movement.
While you could get by with 2 inches of memory foam in a mattress, 3 inches is a plusher and a more comfortable feel for most people. It’s a nice depth of foam where you wouldn’t sink uncomfortably into the mattress or lose the support of the base foam.
Memory foam mattresses do well as, if not better than, other mattress types except for latex. Motion isolation and pressure-point relief are above average. The important thing to remember is to make sure the base is good and solid for your back and neck support.
The complaints that I have heard are about off-gassing odor. Some other complaints are heat retention, excessive firmness, firmness unpredictability, “squishy” feel in high temperature, poor life, unsuitability for romance, and lack of edge support. As for heat retention and other heat-related problems, most have been addressed in your higher to middle models with gel. As far as the long life of memory foam and your romance department, they have done nothing that I know of to help you out there.