I love CM cruises. I'm not saying that because I'm hosting the Carnival Breeze cruise.
I've been on a number of them and I have always had a great time and met many new friends and some "old friends" for the first time and reconnected with others who I have met before.
CM cruises and other group cruises offer amenities you would not get on other cruises and often have a theme that interests people such as music, games, activities and other interests. This means there will plenty of like minded folks on board.
I do have to say there are some groups that can really bug those who are not part of the group. I was on a Princess cruise where there was a large group of "Barbershop Quartets". I thought I was in hell some of the time. There were groups practicing in the community areas, the pool area and in the halls. Barbershop is nice for about two or three songs after that it is like fingernails on a chalkboard. Had I known they'd be on board I would have cancelled. I can't say they ruined the cruise but I found myself trying hard to get away from them and sitting in places and doing things I reaaly didn't want to.
The American Girl group really did shut down booking for the Eclipse, Thanksgiving cruise. Once the word got out there were cancellations and people who were on the fence "Just said no". They were afraid of the extent that 500 8-12 year old girls plus parents would take over venues. I could have cared less but it others did.