My wife and I were on a Baltic cruise and many cities were served by Stromma City Sightseeing buses. I spotted on the Stromma website that if you are doing multiple tours in different cities, if you keep your tickets, hand three tour tickets over and you get the fourth for free. So we paid for tickets (all bought online before leaving home at a discounted rate) for Oslo, Helsinki and Stockholm and got the fourth tour (in Copenhagen) for free, which meant the relatively high price of the tickets became much more reasonable.
The down side to using these buses is that you often lose time in your day waiting for the next bus, some of the services finish mid-afternoon, and to be honest you often have to get off the bus and walk to see the cities properly, but as a means of getting a flavour of a new city they are a handy way of getting an orientation and seeing some highlights. If we have time we usually do a complete circuit on the bus and then use the second circuit to get off at a few places that were on our to-see list. We planned our days in advance using the maps and timetables on the Stromma website and all worked out pretty well.