The Good
Memory foam bed toppers.
I cut 10 inches clear across the bottom of mine figuring the feet didn't need the padding.
Wife thought otherwise so I now have full sheet at my disposal.
The Bad
Memory foam works well as a topper for motorcycle seats (above gel) but doesn't do too well in car seats...they flatten too much.
The Very Bad
Since 1 layer is too thin, 3 or 4 layers must be perfect...It is quite a wrestling match to get the foam in w/out tearing. When done the seats look like they are air inflated (if you do not notice the tear in the cover).
They feel great when you first it on the seat but over time you sink in. To make it worse, nobody sinks in evenly so there is a listing issue to deal with.
I think the foam would be good as a top layer only.