My parents took a few cruises. My father passed away in '11. A year or so ago, my wife and I decided that my Mother would enjoy getting away for awhile, so we took her on a cruise. My mother enjoyed it, my wife tolerated it, and I disliked it.
I know lots of people that take one or two cruises a year and when they get back they can't wait to plan the next one. But it's not for me. As I said above, I like to wander and really experience a place. Everything on the cruise was "artificial" and when we got to port, they tried to funnel us into touristy "excursions." The only time I enjoyed the trip was when we were off on our own exploring the villages away from the ports.
The biggest surprise to me was the food. Most "cruisers" rave about the cuisine on these boats. My wife and I really enjoy fine dining, so I did a lot of research before I booked and made sure to pick a boat/line where the food was especially well-reviewed. It turned out to be mediocre at best and a big disappointment. People around us were all enjoying it, so I think that most go into it with different expectations than we had.
We had a similar experience when we spent half a week at an all-inclusive resort in Jamaica (We were there for a destination wedding). There were tons of people around us sucking down cheap booze and having a grand time, but we ended up just counting the hours until we could get out of there.