Hi Novosbed and Loiroi,
I’m always happy if the retail or manufacturing members here respond as long as it’s not “promotional” and is in response to specific comments or questions or is more “generic” information that isn’t specific to a manufacturer.
[quote]In our opinion (and I believe that Phoenix will agree with me), one of the most important properties to look for when shopping for a mattress with memory foam is its density. Density goes hand-in-hand with longevity, but also affects how far you “sink-in”. Firmness, as measured by IFD (“Indentation Load Deflection”), only measures the amount of force required to depress the foam by 1". An average-sized sleeper on a low density memory foam will “sink in” more purely by virtue of the fact that there is less material between him/her and the surface beneath the memory foam. The same average-sized sleeper on high density foam will sink in less. All Novosbed mattresses use high-density foams, and we almost never have sleepers complaining about “sinking in” too much, or having difficulty turning over - especially on our medium and firm mattresses…
the last comment has piqued my interest. So the density level of the foam plays a more important part in determining firmnrss level.[/quote]
Actually while this can “sometimes” be correct … it certainly isn’t always the case. While the density of memory foam is certainly the single biggest factor in durability … density isn’t directly related to the firmness, “feel” or properties of memory foam and higher density memory foam can be softer than lower density memory foam or vice versa. For example with the Novosbed the medium firmness level uses 5 lb memory foam in both layers while the firm uses a combination of 4 and 5 lb memory foam. Different types or formulations can also have different properties as well that are independent of density (such as greater or less temperature sensitivity, breathability, faster or slower response, or different firmness levels). There is more about the different properties that can be formulated into memory foam in post #9 here and post #8 here.
I would agree that with most of the more modern memory foam mattresses that have a faster response that changing positions isn’t “really difficult” but depending on the specific formulation of the memory foam and on the thickness of the memory foam layers and the other layers and components of the mattress it would still be more “motion restricting” than more resilient materials. For some people this is part of the reason that they prefer memory foam and for others it’s part of the reason that they tend to prefer more resilient materials.
There is more about mattresses and memory foam and sex in post #2 here. You certainly won’t find any agreement or consensus about this but it would probably be safe to say that there are more people who would tell you that more resilient materials such as latex and polyfoam and innersprings that have more resilience or “bounce” would probably be their preference over less resilient materials that have little to no bounce such as memory foam but you will also find a significant and vocal minority that have completely different preferences and love their memory foam mattresses for sex as much as they do sleeping … although it may have taken some experimenting or getting used to 
Phoenix