We did A LOT of research when we bought our new mattress 4 years ago. We had to replace our second queen set in 16 years due to settling and dips. My dh has a bad back, and the mattress made it substantially worse.
Basically what we found was that it is really difficult to purchase a decent mattress w/o spending some decent cash. With mattreses, you really get what you pay for. We also found that with basically ANY spring mattress, you will get settling and dips in the mattress which can aggrevate back problems.
For us the decision came down to a high quality spring or spring/foam top, and deal with settling, or foam...then which brand.
We ended up purchasing a Tempurpedic for around $1800. Nowhere near the top of the line, but we did purchase the name brand. It was around the same price we would have had to pay to get a decent spring mattress, at the time.
I have to say, purchasing the Tempurpedic, although it was tough to spend the money, was one of the best purchases we have ever made. My dh has many fewer issues with his back, and my hips no longer hurt when I get up in the morning. We both sleep great, as there is very little motion transferred when the other person moves, and you tend to move less in general during speep, because there are no pressure points from springs. And, in 4 years, there is no sign of settling, and no need to flip the mattress, which is a big help to me because our bedroom is really tight space-wise.
Granted, the Tempurpedic isn't for everyone, but I would not hesitate to purchase another if I ever need to replace the one we have now.
When you are purchasing a mattress, you have to really consider how much you are willing to pay for a good night's sleep, and how often you are willing to replace your purchase. Do not forget to figure replacement cost into the equation.