I purchased the 4" gel memory foam  mattress  a couple of weeks ago.  So far, it's been great.  
I have a super single waterbed that has ridges that are painful to my back. I needed a cushion so that I cannot feel the ridges.  After letting the foam air out for a few days,  I cut the foam to fit the waterbed (48" x 84").  There is enough cushion so that I cannot feel the painful ridges from the waterbed bladder.  As a result of the foam being on a waterbed, when I lay on the mattress, the edges of the mattress  rise like a hammock.  It may not look beautiful at night, but there is total comfort. I press the foam back into place in the morning.  I tested the foam using only a sheet, and it was fine.  I have since purchased a thin, waterproof mattress pad to keep the foam clean. I have not noticed any smell from the foam. I will monitor to see how the foam responds to the waterbed heater.