Has Anyone used the Hop On Hop Off Buses to see the city while on a cruise in place of standard shore excursions? Are they enough, or will we miss too much by not getting off the "line."

I'd say you'll miss everything if you spend your time sitting on a bus. I'd choose a couple of sights to see in each location.

Helsinki: We used the hop/on hop/off bus and walked around the city.
St. Petersburg: You need to be on an organised tour to see the city which you can sign up for on the cruise.
Riga: It's very close to the port and all walkable. It's a lovely city.
Danzig: It's also walkable.
Stockholm: We used the hop/on and hop/off bus and walked around the old city.

Sounds like you need Rick Steves' "Guide to Northern European Cruise Ports". I never take hop on hop off buses myself, they're very expensive and the local public transport in European cities is usually fine. Helsinki, for instance, has excellent trams.

Which cities? And are you on the roll call for your cruise at cruisecritic.com?

They're expensive but sometimes the audio guides are good and if you don't have time to research, they certainly can be a big help in seeing a city.

The trouble is that you are looking AT a city through bus windows - but actually going into some of the key sights and seeing somethings.

nytraveler: sometimes that's all a person wants. You can get off at any of the stops and then hop on later. There really are some advantages for those who don't have a lot of time to spend in the city or would first like to get an overview of what the city is about.