Our Select Comfort bed is a bit over ten years old and -- although we like it a lot -- it's starting to show signs of age, despite the twenty year warranty. About a year ago, we had to get a replacement set of "box springs", because the plastic had literally bent to the point where the box spring wouldn't stay together. We also got replacement foam sheeting to replace the sheet that had torn down the middle of the king-size bed.
Tonight, we replaced the foam rubber bolsters on either side of the bed, which had pretty much given up any pretense of providing support. The bed is much more square now.