The bunk and couch become a normal sized twin bed, so yes, they are comfortable for and adult. As above, there is a weight limit on the bunk of something slightly over 200 pounds (I don't remember the exact weight, but it is definitely over 200, less than 250), and the adult would have to be willing to climb up there.
The room/drawer space is limited in all the cabins. There are a few tricks that help. First, after the safety drill you take the lifejackets out of the closet and place them under the bed. This gives you an additional large shelf in the closet. Many suitcases will slide under the bed also, but some are too tall. A shoe holder on the bathroom door provides a place to stash many "essentials"...think camera, sunscreen, etc. (not just bathroom needs). Your room steward will provide as many extra hangers (wire variety) as you need. Most of us take way more than we need for the cruise. Shoes are a real bugger--I use black sandles for dress shoes (as well as for casual.) Wear my sneakers, and take a pair of beach/pool shoes. Sometimes I add white sandles. Just eliminating the dress pair saves space/weight.
As to that large shelf in the closet--you can use it for a couple of suitcases not unpacked which holds more than putting loose things there. Also, I leave my "beach stuff" in a suitcase under the bed--that way, it doesn't clutter up the room. I also have a fold up suitcase (actually folds to about 6X 9 X 1/2 ) that I use for stuffing in dirty laundry. This also slides easily under the bed (sometimes hard to get my teen to remember where it is, but.....)