Generally speaking, studies have shown that people suffering from chronic back problems prefer medium-firm to firm mattresses. However, people who don't have a history of chronic lower-back pain seem to prefer medium-firm mattress types.
Most experts advise potential buyers to test the mattress for at least 10 to 15 minutes before purchasing it. The mattress should support the body without putting pressure on the spine, while also holding the buttocks, heels, shoulders and head in proper alignment. The best indication that a different mattress is required is if the person wakes up with lower-back pain that recedes after about 10 to 15 minutes. This could be caused by the mattress being either too soft or too hard.
The best mattress is one in which the user feels very little to no pressure. Mattresses that conform to the body, as advertised by many memory-foam mattress companies, are generally the best at accomplishing this conformation. Experts also suggest purchasing a mattress with a dial-in firmness feature that allows users to change the firmness of the mattress to fit their specific needs.
Ultimately, there is not a general consensus on which mattress firmness is best. Most experts are able to provide an opinion based on clinical studies. However, they also suggest that its best for people to try out many different kinds of mattresses in order to find which level of firmness suits them best.