We had the same issue with a Simmons dual side pillow top. After about 3 years the bed killed my back and had huge valleys in it. The reseller admitted it was a common issue and they would send an inspector out but there was no way it had dropped 1.5" like the warranty required. We trashed it and bought a Tempurpedic... After having 3 back surgeries in recent years I was ready for a decent bed. It seems to have really helped as I feel much better in the mornings. I know there are varied opinions on these foam beds but it did the trick for us.

I was sick throwing out a $2K+ bed that wasn't even 5 years old but glad I did.