If you have lower back pain then you need a firm mattress! Most everyone thinks that, but there is no evidence to support that thought, there is no one mattress that works for everyone, it is really dependent upon your body-type. I get asked this question frequently and the answer is not just a quick name of my favorite.
I found one study in the 2003 Lancet looking at mattress firmness that used 313 people with chronic lower back pain. Half of them were given a mattress of medium firmness, and the other half got a firm mattress. After 90 days of use the medium firm group had less pain in bed, less pain on rising and less daytime pain. Another study in the 2008 Journal Spine compared a Tempurpedic foam mattress, water bed, with a firm mattress and found a decrease in back pain with the foam and water bed mattress.
Earlier I mentioned body-type as a factor in determining the firmness of the mattress. It comes down to posture. Your goal with sleep posture is the same as sitting and standing, to maintain a neutral spine. That means if you are standing your ears are over your shoulders, shoulders over hips, hips over knees and knees over ankles. That is a neutral spine and the position you want when you are sleeping. If you have wide hips or shoulders a medium firm mattress would be better so that the hips and or shoulders can sink so that your spine is straight.
Many manufactures provide a satisfaction guarantee for 90 or more days so that you can return the mattress if you do not like it for any reason. Unfortunately, there are some that have a guarantee but you have to have the manufacturer send someone to check the mattress to see if it has worn down faster than their specifications, possibly leaving you with a mattress that aggravates your back.
The take away here is to make sure you get a satisfaction guarantee for at least 90 days, and unless your hips and shoulders are narrow a medium firm is a good place to start.