I'll admit I have been on one. We went for a week's cruise up the Norwegian fjords for our 25th anniversary. To be honest, I though I would hate it, and I didn't. It's as far from the idea of a peaceful holiday as I can imagine, but I relaxed into it and had a moderately good time. The ship was the Azura (3100 passengers, 1000 crew, size of a small city) but run by P&O, so everything on board was British, from the food to the currency to the general vibe. I don't think I could have stood 30 seconds on an American-run ship.
The food was excellent (I mean really excellent) and the service was friendly without being obsequious, which is as I like it. The organisation was superb, security was tight but unobtrusive, and it was all very professionally-run. The fjords were spectacular, and I will never forget sailing up the English Channel at night with the curtains open, lying in bed and watching an electrical storm over N France and Belgium. We opted for independent dining, so we could choose where we wanted to eat every night, and could choose not to sit with others, which we did most of the time. No dressing up required, although you were expected to be tidy, which is not unreasonable. Some were in jeans and t-shirts and no one said a word.
Overall, a fun if expensive week, totally out of my usual run of things, and I don't regret doing it. I wouldn't go on another, however. Once was enough. One thing it did do was leave me with a burning desire to explore Norway - but under my own steam from the landward side, not in 2-hour bursts from a berth in a harbour, desperately trying to get away from the crowd of fellow passengers and the cruise-ship honey-traps.