that a good "topper" (one that is not as thin as a postage stamp) is very good.

When I hurt my leg...I couldn't even move it...the pain was soooo overwhelming.... I HAD to get one.

I was on vacation in the boonies, so hubby had to go home to work, so he picked up a twin for me (we only have twin beds because of space and difficulty on an island). It did the trick.
I could then sleep.

I think there is one downside to the full Temper...type matress.
That is if you have osteoporosis, or any major other problem,
you can have difficulty moving and getting up from it.
A poster on our PN board mentioned this, and she has developed several pelvic fractures with no injuries... to cause them. She mentioned once she suspected the Tempurpedic mattress was a contributing factor.

They are also HOT in summer or humid conditions.
My topper gets HOT and you can have night sweats.
(easily fixed tho by sleeping NUDE )

Suggestion: The toppers can move around on the mattress, esp if you are a tosser/turner.
Tossing around is less because of the comfort, but some people do move around anyway.
Those waffle mats sold to put under throw rugs...the rubbery ones, placed between the topper
and your mattress will stop sliding movement.
We have an old mattress too, but it is VERY firm. So there are no sags even now.
But it is just too hard now for my pain issues. The topper works very well.

We have them on all the beds up North now..since it is cheaper and easier to upgrade an old
bed with them. It is a horrible task getting furniture or any large object out to our place.
So upgrades are very attractive to me there!