Right off the bat I'll tell you RCI's Serenade of the Seas is a fine ship, well staffed with a cheery easy-going crew and their Southern Caribbean itinerary out of San Juan is just perfect for a mid-winter getaway.
It seems to be a happy ship and I heard it's more popular generally than two other cruise ships doing similar itineraries from San Juan. This ship was fully booked on our January 3rd 2009 sailing but it never seemed crowded. I have no idea where everyone was hiding. Seats where always available and there was never the crush to line up which I've seen on other cruise lines.
The Serenade was spotlessly clean. The banquet class dining included well prepared food, a reasonably broad menu selection and excellent presentation. The crew was friendly and helpful while the entertainment was fun without being too extravagant or loud. Marc Walker the cruise director seemed to keep things moving - funny without being forced or crude. There were not many children to be seen but basically this seemed to be a family oriented cruise with just the right touch for old and young.
Cabin 4555 was close to the Centrum area. Easy to get dining room and elevators. Had a very slight sewage odour in the hallway when maintenance workers were fixing a nearby plumbing problem.