I've had it about a month and so far it's been great.  I left it alone the full three days that were recommended and it really does matter.  It was at 9" for about a day, then 9.5" for about a day and finall the full 10" by the third day.  And the mattress feel changed over that time as well.  So, yes, they say it for a reason
I'm about 5'3'' 115lbs and this med-firm is just right for me.  It felt maybe a bit too firm the first few days, but now it's perfect.  And honestly, even if this mattress only lasts a year, IDC, it was a great price and I would buy it again next Way day