I purchased these to place under my box spring.  My husband has a bad back and for the last month or so, he's slept on a mat on the hardwood floor because he just couldn't sleep through the night on our bed.  
I used these (along with the standard version placed between the mattress and box spring) and I am telling you:  He has slept through the night and awakened refreshed - no sore back, no tossing and turning, no more sleeping on the floor.  With both bed slats it is like a new bed - very, very firm.  No complaints.  I recommend to anyone with problems sleeping through the night.