We've taken about a half dozen of them and like them OK, although we've been talking about the fact that we want to do some cruises that we haven't done yet, especially to South America.
Usually we use repos as transportation to get somewhere we want to go, or to come home from same. I'm always a bit stunned at the huge numbers of people who fly to the cruise, sail immediately, then fly back home on the day it ends. Sea days are nice, and we enjoy them too, but there's a big world out there.
We just took a repo to Southampton, then flew across the Channel to Guernsey so I could continue my family history research. We've always got some sort of agenda associated with a repo.
The well-known expert-of-the-seas and frequent cruise lecturer John Maxtone-Graham has always maintined that the vibe tends to be entirely different on crossings among the people who are "going somewhere." He claims that the shared excitement about what's at the other end (as opposed to a four hour tour at a port stop) creates an atmosphere that's totally different from, say, a Caribbean cruise.
I think he's right to a certain extent, although with so many people just cruising for the experience of being on the ship, sometimes that excitement is a bit muted. It's also obviously true that on virtually any line the age range of the passengers will skew toward the high side compared to many other cruises.
It's also worth noting that crossings are heavily populated by "crossing junkies." These are either bargain hunters or cheapskates, depending on your point of view. They tend to be elderly, they take crossings more or less exclusively because they generally offer the cheapest per day fares out there. Therefore these people quickly earn top status in the line's frequent cruiser club. They covet these perks and they make full use of them, especially where free alcohol is involved. They tend to not spend for a lot of "extras," and are high on the list of those who complain about "nickel and diming." This can work to the advantage other passengers who enjoy, for example, specialty restaurants, because tables are often easily available on crossings, even for multiple visits.
So, there are plusses and minuses. I think it's important these days to do it on a beautiful ship with good service and interesting activities that match your interests.