From what I'm reading in the responses, firm mattresses are good for a lot longer than the softies, especially the "non-flippable" pillowtops.
Our experience pretty much echoes that, we have one super-squishy pillowtop King that's still great after 2 years but also a super-squishy pillowtop Queen in spare bedroom that's 10 years old and has a cavern that makes me wonder when Duke Caboom is gonna show up to try and jump it.
But DW says the firm mattresses hurt her back, so we'll stick with the super squishies and just deal with the PDC (People Dent Cavern) in the spare bedroom for the time being as it's not used regularly.