I've done half a dozen RC cruises in the past few years. My opinion: totally ship-dependent. I have no complaints with the clientele (funny story below)...the food has ranged from good to outstanding with no middle ground. Same thing with the service. So it really depends on the ship. My family and my wife's family are big cruisers, they prefer Princess, but remember they are a bit older (40s and up). Another of my sisters really likes Carnival, and I understand its reputation has gotten better since the merger a few years ago. But even the worst of the cruises I have been on would be very impressive for a first-timer, it's just that once you have enough experience to compare, some seem better than others.
Funny story: we went on a RC cruise once, and noticed the first night at dinner that the dining hall was only half full. I asked the waiter and indeed, the ship was only half capacity. General Motors had booked the ship primarily for its Mr. Goodwrench Master Mechanics, so the passengers were pretty much blue collar. We had no problems at all on the cruise, but on formal night one rather overweight gentleman wore a tuxedo T-shirt. It was hard to keep from laughing.