I purchased a Serta I comfort 8 months ago. I started to sag in the middle.
I called the store -mattress warehouse- and they sent out a representative. He looked at it, measured the sag and agreed that it was defective. He then proceeded to tell that since I had a "spot" on the mattress cover my warranty was void. This "spot" is the size of a dime.
In order to see it you have to zoom in with a camera to the point that you can read the date on a dime.
Beware of Serta and mattress warehouse. Mattress did not last a year....
Monetary Loss: $1150.