I got an Otty last year, reason I went for it was because all the reviews said it was firmer than the other box mattresses on the market, which was the most important factor to me.
I left it for two days myself (luckily there was a bed free in the spare room for the duration), but that was almost entirely because it was absolutely honking. Not sure if I was just unlucky or if I happen to have a nose sensitive to whatever aromas it contained.
It’s been nearly a year now, and I’m still very happy with it. Still comfortable, haven’t noticed any problems with it, and the pillows that came with it are decent too.
My only quibble would be that I wish it had handled on the side to make rotating it a little less faff! But I can live with that really - the sleep is the most important part, after all.