Before you buy, ask the store what their return/exchange policy is.
In fact, I would call and ask several stores before I went in to shop.
One store here said that after the mattress is delivered, you must keep it for 30 days before they will allow you to exchange it.
I guess they think that by then, you will get used to it and won't want to send it back.
But what are you supposed to do for all those nights if the mattress turns out to be so uncomfortable that you can't sleep on it?