Not my kind of thing, no (for one, I would not want to be on a ship for that length of time with that many people), but I know several older couples who have gone on quite a few of them and really enjoyed them. In fact, they book them months and months in advance. And it's not the party crowd. Buffets are open 24/7, there's a lot of jazz music, gyms, running tracks, etc. But even so, yes, it's easy to run up a big bar tab that you're hit with before you disembark.
If you can't pay it, I don't know, maybe they keep you.