I have had my real deal Temperpedic mattress since the late 90's (probably close to 20 years). I have no indentations on it to this day. Yes, they recommend rotating and or flipping the mattress occasionally. (This reminds me I should do that soon).
I don't know how firm you want it to help your back, but for me the more firm the better. Shortly after buying it I put a piece of 3/4 inch plywood between the mattress and box spring across the center 1/3 (basically above the knees to the middle of my back). I never have back problems. I was sleeping on my GF's bed (also a Tempurpedic) and was having back issues. I put a 3/4 plywood sheet on my side only and the back issues went away.
I don't know how I would have felt if I had paid full price for it (my then wife worked at a store selling them and got it at a huge discount
), but I have never regretted getting it.