Actually Morten said it all. We did the same cruise last May. It's beautiful and you definitely want to spend the first 4 hours of the journey on deck as you'll be passing a lot of sights. Beer and soft drinks are expensive but not unlike anywhere else in Scandinavia. The cabin sizes are ok - quite normal for such old ferry boats. Try to pack in a way that whatever you might need for that one night is handy on the top of one piece of luggage - so you can e.g. hide everything else underneath the bed or so.
People do dress up for the night club on board. So if you're into that type of socializing bring some cool clubbing clothes as well.
The coffee on board the next morning was quite good and not more expensive than at your local Starbuck's... Have it on deck while all the others are still sleeping and enjoy the 2+ hours sail into Oslo Fjord... Also do stay on deck until the vessel has docked in Olso. It's the best place in the entire city to take picture of the Olso Opera House.
Another cool cruise on an even better ship is Fjordline from Bergen (Norway) to Hirtshals (Denmark).