I've done several 4 and 5 night cruises solo and loved them. I'm about to embark on my longest yet, 9 nights onboard with 1 night pre-cruise and 2 night post-cruise hotel stays. Really looking forward to it. As others have said you can set your own schedule and then change it at the last second if the mood strikes. While I enjoy eating in the main dining room when I'm part of a group, on my solo trips I'll usually just hit the windjammer so I can get in and out. It is what you make of it. Everyone else is on vacation and generally in a great mood so it's easy to strike up conversation if that's what you want, but also easy to find some peace and quiet and just take in your surroundings if that's what you want as well.