We have been to CN 2x, and if you haven't yet, I second the suggestion to go to whe message boards at www.couples.com .
Both times we stayed in the lowest level rooms, and thought it was great. Nothing crazy fancy, but we did not spend much time in the room, and would prefer an extra night versus paying more for the room. That's the great thing about Couples, everyone gets the same service no matter which level of room you have. We had no interest in the suites. The only reason I would want to pay more would be for a true ocean front view, and there are only a very few rooms there with that.
I thought the beds were great, very comfy, and I have a history of back problems, but I realize that's very personal and every back is different.
However, I will say, if you haven't booked yet, consider Couples San Souci. We went there last time, and got a Penthouse suite for cheaper than the lowest level room at CN. We loooooved it. The beach is not quite as nice as CN, and there are lots of steps, but we thought everything else was better. (which is amazing since we really, really love CN.)
You can't go wrong with Couples, though. Have fun!