We had a pillow top that we didn't care for. Went to Overstock and bought a 3" Serta Memory Foam for about $125.00 with $2.95 shipping (Cal King). Loved it and that was 6 yrs ago and it's still holding up. Bought one for parents-they loved it. Sis bought one-she loved it.
All in all, it makes the mattress a total of 14" high so I just make sure to buy deep pocket sheets.