My parents loved their first Viking trip but subsequent ones have been problematic. Viking changed some ports late on one. They also did one in Europe where the river was too low to have everyone onboard so Viking would bus everyone between ports and then they would board to eat/sleep.
Except for the low river one, I think they've generally enjoyed Viking for European trips and had bad experiences with them in North/South America. I guess I'd probably see when the river's low season is before booking one.