My family recently took a Viking cruise from Basel to Amsterdam. Our group was made up of myself and my fiance (28 + 27), my parents (55 + 57), and my Grampie (79). We decided on this cruise because it had been a trip my Grampie had wanted to take for years, and we wanted to make sure he got to take it.
Generally, this kind of vacation would not be what my fiance and I or my parents would choose for ourselves. We prefer to spend more time in each city, do less tours, get to know the area, etc. My parents prefer to relax and not really explore. However, we all got something out of this trip that we didn't expect and honestly loved it! My fiance and I were well and beyond the youngest people on the ship, and these tours truly were not designed with our age group in mind - so if you're around our age, I would suggest looking elsewhere unless you're traveling with family. My Grampie loved the pace of the trip and only got tired/sore because we dragged him on a full-day walking tour in Strasbourg, which the rest of us loved but he probably could have done without.
The fact that the itinerary was set meant that there was no tension or bickering about what we each wanted to do each day. We never had to worry about what cafes were open for breakfast, where we could stop for lunch, etc - meals were all planned on the shi[ and the food was good. We also loved that we only had to unpack once and that we got used to our stateroom; very different from packing up a bag in every city. I really enjoyed this aspect of the trip.
The rooms are tight, but not claustrophobic because of the floor to ceiling sliding glass doors. We loved having the french balcony because it meant that even if we were taking a bit of a mid-day rest in our rooms, we could still watch the world go by. I also HATE air conditioning and being able to open the door to get fresh air was a lifesaver. It's also good to know that you won't need adaptors because they have North American style plugs.