Our two year old mattress had a "dip" in the middle making the bed very uncomfortable.  So far, one week into using the mattress topper, the mattress is much more comfortable as it is now flat (i.e. dip is gone).  I hope that it lasts.  There is a slight problem as the topper shifts during the night, making it a little "lumpy".  I set up the topper with the flat side on top.  Is there a better way to set up the topper so that it doesn't clump up?