My mother and aunt have been on two and loved them. They have also been on an Alaskan cruise. they much prefer the smaller River boats. The ships are not crowded, the staff were lovely. Mum did get tired of yet another excursion to a church, so she and auntie did their own thing some of the days.
She would love to go again.
Crowds will depend on time of year, how many boats are berthing at a time etc. I spent last May in Florence. At the beginning of May it was not too busy, by the end it was incredibly crowded with big tour groups.
I am spending most of this May in France, but at some point in the next 8 years I want to take a river cruise in Europe. I would go in May if possible.