It is true that the biggest complaint against memory foam has been a 'sinking feeling' - therefore not being able to move freely whilst lying down - and that it tends to 'cook the sleeper' because body heat is not dissipated quickly.
Simmons claim to have solved both of these problems using their version of foam called NxG Advanced Memory Foam. From what I've garnered there does seem to be a small reduction with the quicksand feeling many customers complain about.
With the claim as to heat reduction, I can't say one way or the other. The one overriding opinion is that the Simmons ComforPedic mattress is cheaper than a Tempur-Pedic mattress, though a little softer, and offers good value for money.
Simmons makes the following models of ComforPedic mattresses: The Simmons ComforPedic Original - comes with a removable, professionally dry-cleanable velour and suede cover.
The Simmons Nassau - comes with an additional layer of latex.
Simmons ComforPedic Natural is the next step up - you get a thicker layer or memory foam, an inner-layer of latex and the Simmons patented Edge Support System (some people do like a firm edge to their mattress though how this offers more support whilst sleeping is beyond me).
Simmons Mystere - comes with the thickest memory foam layer, otherwise it's the same as the ComforPedic Natural.
Simmons ComforPedic Nuvo - this is the most expensive and comes with the Simmons Nuvo bed and allows custom positioning of the mattress, which can be programmed to an array of positions.