I had a "Select Comfort" bed, which is what Sleep Number was originally called (left it behind in my divorce). Slept on it for almost 10 years without ever having a sore back in the morning after living 20 years with a stiff, sore back. I replaced a waterbed when I bought it and my ex pitched a fit about it until I showed her the seperate controls for firmness (kingsize bed). The cost isn't really that much more than a top of the line mattress and the Select Comfort can be replaced piece by piece if something wears out or gets damaged (things very rarely break). The pillow top zips on and off, the air chambers can be replaced (I had a seam fail because the mattress was in a waterbed frame and wasn't completely deflated when the sheets were changed, stressing the seam.)
I'd recommend one in a heartbeat.