Sealy Posturepedic Mattress Review

We purchased our Sealy Posturpedic mattress 8 years ago and in the last 3 years both sides of the bed of slumped down deeper and deeper. During the time of purchase, we were told by the sales rep, that this mattress would not cave in and we were guaranteed the utmost comfort. When we lie in bed we both roll into the middle of the mattress. We are now on the hunt for a new mattress and have no desire to go back to Sealy. Any recommendations you may have, I would be very interested in hearing from you.