My wife and I both thoroughly enjoyed this cruise, this vessel, and Russia. We made changes to our Viking-supplied air and had no trouble. (Another review takes great pains to spell out their horrible upgrade process but that's on Aeroflot, not Viking.)
We arrived Moscow a couple days early in order to explore on our own. All went well. The cab ride to the port was without drama except for not understanding the driver when he was asking for the name of our ship. The port was LOADED with river cruise ships. I gave him the name when it seemed appropriate and he got happy.
Embarkation could not have been easier or quicker: a trailer contained a scanner for our luggage and a security person gave us a look. Done.
Saw it on our own and as cruise excursion; great to visit an historic place; Saint Basil's is awesome; GUM is impressive; Lenin's tomb is surprisingly suppressed.
As we had seen Red Square and the Kremlin on our own a couple days earlier, we used some of the cruise excursion time to tour the State Historical Museum. It includes artifacts from dinosaur bones to carriages of the czars. All placards are in Russian so unless you know the language, it's hard to get a full appreciation. Still a good visit. We also visited the War Museum which is now showing an exhibit of the Great Patriotic War, what Russians call the War of 1812: the attack by Napoleon. It was splendidly done!