I wanted to see Russia and thought this would be a good way. I would definitely recommend the Viking cruise.
The dock in St Petersburg is out in an industrial area so if you want to explore on your own you will need a taxi. I believe it is possible to take a city bus to the Metro station on the other side of the river. There is an ATM in a souvenir shop on the dock if you need Rubles. In Moscow the Metro station is closer to the dock if you want more free time in town.
We were able to see a lot in St Petersburg and Moscow, but the river trip between them really gave me a change to see more of Russia. Unlike the western Europe trip there was more time spent on Russian History, the Russian language and showing us a Russian home, which I found interesting. I was surprised at how open and honest our guides were. They did a great job showing us their country. I returned with a better understanding of Russia and its people than I had before. I felt safe everywhere we went, and really enjoyed the trip.
Cabin was small but comfortable. The veranda was especially nice for the cruising part of the journey.