Yes, I did. I had read that it might be difficult to get a one day pass on board, so I prepurchased it.
I used the form that is provided and faxed it to DCL. When we got to our cabin the piece of paper was on our desk with all the other paper work like excursion tickets and dining tickets. It's just a plain white sheet that says rainforest one day pass and the number of days we had bought. We had two, one for each of us.
When you want to redeem it, you just take it to the spa and they put a little sticker on your KTTW card to show that you have the pass for that day as you can come and go as often as you wish.
We were on the Wonder and there the room has 4 heated tile loungers, I think three different warm rooms (One with steam and two others) and thre different showers. There is scented water and tea available and you get a robe in the dressing room. You are required to wear your swimsuit. The rainforest room on the Wonder and magic is all inside, no windows.
On the newer ships there is also a small hot tub and there are nice large windows from the pics I have seen!
I liked it, but it was not as wonderful as I thought it would be. Especially when more people arrived and I felt that they were waiting for a tile lounger I could not really relax anymore.