We've been on two cruises: Rhapsody last year, and Mariner last week.
We found that the beds were very comfortable on both ships. Mattresses were firm, which is a plus (spongy, bouncy mattresses leave me crippled in the morning). We slept very well on these beds, as did our daughter and her friend, who napped every single day and one evening even sacrified dinner to sleep.
I wasn't thrilled with the somewhat stiff pillows on Mariner, but it wasn't a big deal. Just a matter of preference. I prefer well-stuffed down pillows. Had we booked a suite, that's probably what we'd get. But it didn't impact my sleep.
Enjoy!