We have a three year old mattress, king-sized, and were constantly having problems with our lower back. Not wanting to buy a new mattress, I tried this pad.
I managed to keep using my normal fitted sheets, and although it was a tight fit, our lower back problems ceased. For the $80 (with shipping) that we paid, it has been more than worth it. I would recommend this to anyone liking a soft, more comfortable rest, along with back support.