When my old mattress was thoroughly dead (as in - there was no longer any spot you could lie and not feel at least one spring) I took temperpedic up on their offer of a 4 month in home trial, loved it, returned it, payed nothing.
Then I bought two generic extra thick memory foam toppers, about $100 each I believe, and stacked one on top of the other. One is a little more dense than the temperpedic, the other is a little less dense, so I can change the feel based on which one is on top.
OMG, it is more comfortable than the Brand name. I can't believe anyone still sleeps on steel springs now that vicso-elastic memory foam exists.
Haven't found any sex related problems what-so-ever (though granted, we have a total weight of about $260, so it may be different for others? then again, I had the same mattress when I was single and dating, and oh boy that was an adventure......)
As for longevity, its been about 8 or 9 years so far, and it hasn't degraded at all yet.