I used to work for Royal Caribbean on the Vision of the Seas, and I think cruising can be fun depending on what sort of vacation you like to take. If you like all-inclusive, structured activities like games at the pool or dance classes, or if you like Las Vegas or ESPN Zone or the Hard Rock Cafe, then you'll probably like cruising. You can definitely go off on your own and do your own thing, but they make a big effort (on Royal Caribbean, anyway) to provide stuff to do all the time.
If you don't enjoy getting to know the the people around you, or you just want to sit quietly by the pool without having to watch a belly flop competition or a calypso dance class, cruising is probably not for you. If you're the sort of person that likes to explore on your own and take time to get to know a place, cruising is not for you. You're usually only in each port for a day, and sometimes as short as 5 or 6 hours.
On Royal Caribbean, the shorter the cruise, the younger the crowd. A 3 or 4 day cruise was pretty much a booze cruise with tons of people in their 20's. A 5 or 7 day cruise would be a mix of families and young couples, and a 10 or more day cruise was mostly old people. (I remember a 14-day Panama Canal cruise that had 3 children on board out of 2,600 passengers.)
I've cruised twice as a passenger to visit friends that are still working on board ships. One of those was a 10-day Med cruise. We went from Spain to Sicily, Turkey, Greece, and Malta on the Navigator of the Seas. It was nice, but DW and I are more of the explore on our own type, so I wouldn't pick cruising as a first choice.
Hope that helps. I was on the Vision for almost 3 years, so feel free to ask me anything.