It is shape memory but seems to still settle and change shape quickly enough (a few weeks). As guaranteed long enough, I would see the holding on a little longer.
Otherwise, a little more firmness would have made it more enjoyable. I find it a little soft.
When using the extension tube, after a few days, one must remove in order to properly recover the parts supplied in the pillowcase. Not very practical.
This still remains a good cushion despite a quality / price rather bad.